JPH0344495B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an overcurrent detection device that detects a fault current in an electric circuit and enables optimal protection.

[Conventional technology]

Generally, in a static overcurrent detection device equipped with a microcomputer, for example,
As shown in Japanese Patent No. 159922, detection characteristics (protection characteristics) against fault currents in electrical circuits are configured to be obtained by executing a predetermined program written in a ROM in a microcomputer. The above-mentioned detection characteristic is usually designed to provide an inverse time limit characteristic as shown in FIG. 1, for example, in order to protect the electric circuit and the load. Generally, the detection characteristics as shown in FIG. 1 are set in consideration of the heat resistance of the distribution line, the fusing characteristics of the upper fuse, and the like.

Conventional devices of this type include current sensor means for detecting fault current, means for sampling the detection signal at a predetermined sampling rate, means for determining the level of the detection signal, and a device corresponding to the level as shown in FIG. It was configured to include means for performing a time-limiting operation based on inverse time-limiting characteristics.

Even in the conventional equipment described above, the heat dissipation characteristics of the electrical circuit and load are taken into consideration in order to respond to cases where the fault current changes or when the fault current returns to a normal state within a set time limit. It is possible to store heat dissipation characteristics in the program.

It is generally known that the heat dissipation characteristics of power distribution lines and loads have such characteristics that the temperature decays exponentially. Therefore, by calculating within the microcomputer based on this attenuation characteristic, or by having already known calculation results as a data table in the ROM, it is possible to respond to fluctuations in fault current, etc. The device can be realized.

[Problem that the invention seeks to solve]

However, when performing processing that takes the above heat dissipation characteristics into consideration in the program,
The following problems arise. That is, when calculating on a program, a complicated and enormous program is required in order to obtain heat radiation characteristics with high accuracy. Therefore, the processing time becomes very long and optimal protection becomes difficult. On the other hand, if the above calculation results are stored in advance as a data table in ROM,
Although the program itself can be shortened, obtaining accurate heat dissipation characteristics requires a huge amount of data and a large-capacity ROM. Especially when using a one-chip microcomputer, etc.
Since the capacity of ROM is extremely limited, it cannot store enough data and therefore it is difficult to obtain optimal characteristics.

There is also an even more serious problem. That is, if the power supply is temporarily cut off or disturbances such as surges and noises enter, the microcomputer is reset. When the microcomputer is restarted after such an unexpected reset operation, in the conventional device, all processing is restarted from the beginning, and the data stored up to that point is also erased. This is a major problem regarding the basic function of the overcurrent detection device. Moreover, this problem has a particularly serious effect when the secondary output of the current sensor means is used as a power source for a microcomputer. That is, if no fault current or normal current flows, the operating power for the microcomputer is lost, so the above-mentioned restart process is executed each time for intermittent fault currents. Therefore,
There was a problem in that thermal protection, that is, an operation of outputting a detection signal in accordance with the amount of heat generated by excessive current, could not be performed at all.

This invention was made to solve the problems with the conventional devices as described above, and it is possible to obtain optimal heat dissipation characteristics with a small-capacity microcomputer using a simple program, and also to reset the microcomputer. It is an object of the present invention to provide an overcurrent detection device capable of performing restart processing at a subsequent restart or in response to an intermittent fault current.

[Means for solving problems]

The overcurrent detection device according to the present invention includes: a level determining means comprising a microcomputer and determining the level of fault current; a time limit generating means comprising the microcomputer performing a predetermined time limit operation corresponding to the level; an output means responsive to a timed operation of the means; and a pulse generating means for generating a pulse having a pulse width corresponding to thermal energy generated by a fault current at predetermined time intervals during the timed operation of the timed generation means. It is equipped with a pulse generating means consisting of a microcomputer, and a charging/discharging circuit means that is charged by the pulse and discharges at a predetermined time constant, and the charging/discharging circuit means is provided, and the charging/discharging circuit means is charged by the pulse and discharged at a predetermined time constant. The residual voltage of the discharge circuit is input to the microcomputer as an initial value for calculating the generation time limit of the time limit generating means.

[Effect]

The overcurrent detection device of the present invention charges a predetermined pulse corresponding to the thermal energy generated by the fault current as part of the time limit generation process of the time limit generation means consisting of a microcomputer when a fault current occurs. The capacitor that makes up the discharge circuit is charged periodically to change the charging voltage. Next, when the fault current returns to normal, an exponential decay characteristic based on the heat dissipation characteristics is generated using the discharge characteristics of the capacitor, and when the fault current flows again, the residual By reading the voltage into the microcomputer as an initial value, a memory effect of accumulated heat is provided against intermittent fault current.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 is a block diagram showing an embodiment of the overcurrent detection device according to the present invention. In FIG. 2, the AC line 10 includes a converter 20 for current detection.
is provided. A rectifier circuit 30 is connected to the secondary side of the current transformer 20 for obtaining the absolute value of the secondary output. A burden circuit 40 is connected to the output side of the rectifier circuit 30. The load circuit 40 converts the output current of the current transformer 20 into a voltage signal, and
It also serves as a level adjustment circuit for obtaining an output signal within a predetermined level range. The output side of the burden circuit 40 is connected to a waveform conversion circuit 90. The waveform conversion circuit 90 obtains the effective value of the output signal induced in the burden circuit 40. An output terminal of the waveform conversion circuit 90 is connected to a first input terminal 101 of an A/D conversion circuit 100 that converts the analog output signal into a digital signal. The output of the A/D conversion circuit 100 is input to a microcomputer 110. The microcomputer 110 is provided with output ports 117a and 117b. The output port 117a has an output device 80.
is connected, and the output device 80 has an output terminal 81
It is equipped with A capacitor 50, one end of which is grounded and the other end connected to the second input terminal 102 of the A/D conversion circuit 100, is connected to the output port 117b via a diode 70 for preventing backflow. A resistor 61 (for discharging) is arranged in parallel with the capacitor 50. A charging resistor 62 is connected to the anode side of the diode 70.

The configuration of the microcomputer 110 will be outlined based on FIG. In FIG. 3, a microcomputer 110 includes a ROM 114, a RAM 115, and an
It is composed of an O port 116. The output ports 117a and 117b of the I/O port 116 are connected to the output device 80 and the anode of the diode 70, as described above. A portion of the data bus 112 and address 113 are connected to the A/D conversion circuit 100. Generally, the ROM 114 includes a program for executing predetermined signal processing,
The CPU 111 executes a program in synchronization with a predetermined clock signal. Further, the RAM 115 functions as a register necessary for signal processing.

The signal processing process in the microcomputer 110 is shown in the main flowchart of FIG.
This flowchart includes the following basic functions:
It includes at least a level determining means 1001 for determining the level of an input signal, and a time limit generating means 1002 for executing a predetermined time limit operation based on the level determined value. This flowchart also includes accurate heat dissipation characteristics when the overcurrent condition is no longer present, in order to operate the overcurrent detection device according to the discharge characteristics described above and the characteristics combined with the heat storage characteristics. Means 1003 for generating is included.

Next, the operation of the inventive apparatus configured as described above will be explained below.

When a fault current flows through the AC line 10, the current transformer 2
0 detects the fault current using its own current transformation ratio and induces an output current on the secondary side. This output current is converted into direct current by the rectifier circuit 30. The output current of this DC-converted rectifier circuit 30 is transferred to the burden circuit 40.
supplied to The output signal of the burden circuit 40 is converted by the waveform conversion circuit 90 into a signal corresponding to its effective value. The effective value output of the waveform conversion circuit 90 is input to the A/D conversion circuit 100. A/D
The conversion circuit 100 is a microcomputer 110
, and converts the input signal into a digital signal in a time-divisional manner. These digital signals are connected to data bus 1 of microcomputer 110.
12. Microcomputer 110
executes level determination of these digital input signals according to a predetermined program. Furthermore, based on the result of this level determination, a predetermined time-limited operation is performed and an output signal is generated from the output port 117a. The time-limited operation in this case is performed, for example, along the characteristic curve shown in FIG. The output device 80 is driven by an output signal issued from the output port 117a of the microcomputer 110. From the output terminal 81 of the output device 80, an output signal for displaying a fault current, protecting a circuit, etc. is obtained.

In addition, in the processing of the above-mentioned time limit operation, the following processing is also executed. That is, during a certain period of the above processing, the capacitor 50 is periodically charged with a square wave output corresponding to the thermal energy generated by the fault current. This will be explained in detail based on FIGS. 4 and 5.

FIG. 4 is a characteristic diagram showing the state of change in the current flowing through the electric circuit and the operating characteristics of the overcurrent detection device of the present invention. Further, FIG. 5 is a waveform diagram showing how the capacitor 50 is charged and discharged.

As shown in Figs. 4 and 5, during the period T ₀ during which the fault current I ₁ is flowing, the thermal energy that should be generated during the operating time T _{1 corresponding to the fault current I 1 is} calculated by unit time. Periodically (every predetermined unit time), a pulse having a corresponding pulse width t ₁ is sent to the I/O
is generated at port 116 to charge capacitor 50.

Here, the above-mentioned pulse width t ₁ is determined by outputting a pulse having the pulse width t ₁ at a predetermined period, so that the state of charge of the capacitor 50 is reduced to a saturation level of 5V after an operating time T ₁ corresponding to the fault current. is determined by microcomputer 110 to reach . As _a result _{, the voltage α [} V] is charged to the capacitor 50.

Next, when the fault current I ₁ returns to the normal current I ₂ after the period T ₀ has elapsed as shown in FIG. Discharge.

Next, when the fault current flows again, the microcomputer 110 determines the level of the input signal from the A/D conversion circuit 100 and executes the program to perform the predetermined time-limited operation again. At this time, the fifth
In the figure, at time T _S , the microcomputer 11
0 is the residual voltage _VTS of the capacitor 50, A/D
The voltage level is read through the conversion circuit 100 and processed as an initial value for generating a new time limit. 5b shows a charging and discharging waveform of the capacitor 50 using a constant voltage according to this embodiment, and FIG. 5c shows a charging and discharging waveform using a constant current according to another embodiment.

The signal processing process in the above-mentioned microcomputer 110 will be explained in detail along the main flowchart of FIG.

When the microcomputer 110 is started and becomes operational, the program starts, initializes the system (i.e., sets the I/O port 116, sets flags, resets, etc.), and starts the main processing flow for overcurrent detection. to go into. Next, a control operation (A/D conversion process) of the A/D conversion circuit 100 is executed. Through this control operation, the signal of the effective value of the current in the electric line 10 and the voltage signal of the capacitor 50 outputted from the waveform conversion circuit 90 are selected in a time-division manner and converted into digital signals, which are then input into the microcomputer 110. Write to RAM115 of. Next, regarding the input signal data written to the RAM 115 as described above, an operation is performed to determine whether or not the value is an overcurrent value. As a result, if there is no overcurrent, the process exits from the heat storage routine shown in FIG. 6 and returns to the A/D conversion process described above. Next, if there is an overcurrent, the heat storage flag H is first set, and a register in the CPU 111 or the RAM 115 is used to set a predetermined time every predetermined unit time in order to execute a time-limited time measurement operation according to the level of the input signal. The number of heat storage bits is added. The predetermined number of heat storage bits is selected to achieve a time-limited operation along the characteristic curve of FIG. Next, an operation is performed to determine whether the number of bits added as described above has reached a value corresponding to a predetermined time period. Then, the number of bits added above is
When the value corresponding to the predetermined time limit is reached, an output signal is generated through the output port 117a to drive the output device 80.

Here, if the added number of bits has not reached the value corresponding to the predetermined time period, processing is performed to charge the capacitor 50 with an amount of charge corresponding to the value of the overcurrent, and A/D conversion is performed again. Return to processing.

Next, a case will be described in which the overcurrent returns to a current within a normal range. As mentioned above, if the maximum value of the A/D converted data falls below a predetermined level when the heat storage flag H is set and the timer operation is in progress for a certain period of time, it is determined whether or not there is an overcurrent. The process departs from this determination routine and performs an operation to determine whether or not the heat storage flag H, which indicates the state at the stage immediately before the relevant stage, is set. As a result, if the heat storage flag H is not set, the process returns to the A/D conversion process. Next, if the heat storage flag H is set, the heat radiation bit based on the predetermined heat radiation characteristic is subtracted from the count register to which the heat storage bit is added to realize the heat radiation characteristic, and the subtraction result becomes zero. It is determined whether the value has become zero or not, and if it has not become zero, it returns to the A/D conversion process, and if it has become zero,
The heat storage flag H is reset and the process returns to A/D conversion processing.

The program is configured and the process is executed as described above, but the microcomputer 110 constantly reads the voltage of the capacitor 50 during the A/D conversion process. By doing this,
When overcurrent begins to flow, the counting register is started using the residual voltage value of the capacitor 50 as an initial value. Even if a fault current flows intermittently, or if the microcomputer 110 is reset and restarted for some reason, the capacitor 50 will have a residual voltage corresponding to the thermal energy that had been accumulated in the electrical circuit. Retained. Therefore, the device of the present invention can realize an operation that conforms to the heat storage and heat dissipation characteristics of the electric circuit.

In addition, in the above processing, the voltage signal due to the residual charge of the capacitor 50 is processed in each A/D conversion process, but the data is read only at the beginning when the microcomputer 110 starts. It can also be used only as the initial value of heat storage data, as information on heat storage and heat radiation characteristics during a period when the system is stopped for some reason. Further, it is also possible to execute the voltage signal of the capacitor 50 separately in the initialization processing when starting the microcomputer 110, and not to read it in the processing flow while the microcomputer 110 is continuously operating. be. Furthermore, in the above embodiment, the power source for operating the microcomputer 110 etc. is supplied from the current transformer 20, but a second current transformer different from the current transformer 20 is provided,
The operating power may be supplied from this second current transformer.

〔Effect of the invention〕

As described above, according to the present invention, by appropriately selecting the discharging time constant of the capacitor (in the above example, appropriately selecting the value of the discharging resistor 61), the heat dissipation characteristics of the actual distribution line and load can be greatly improved. Very similar attenuation characteristics (operating characteristics) can be obtained. Programming is also easier and requires less storage space.
Arithmetic processing time can also be reduced. Furthermore, an ideal overcurrent detection device with high accuracy can be realized from the viewpoint of providing protection against intermittent fault current according to the heat resistance of the distribution line and load. Furthermore, even if the detection operation is temporarily stopped and then restarted due to disturbances such as a drop in power supply voltage or surge/noise, the heat storage data up to that point is retained, so appropriate protection can be provided. Furthermore, since capacitor charging processing is performed during each overcurrent heat storage process, similar processing is performed only when the capacitor is depleted due to overcurrent (it must be processed before the control power supply decreases).
Compared to the above, the capacitor can be charged with sufficient power, the capacity of the control power supply including the microcomputer can be reduced, and the power supply circuit can be made very simple.

[Brief explanation of drawings]

FIG. 1 is a diagram showing general characteristics of an overcurrent detection device, FIG. 2 is a block diagram showing an embodiment of the overcurrent detection device of the present invention, and FIG. 3 is a diagram showing a microcontroller in the overcurrent detection device of FIG. A block diagram of the computer and its surrounding elements, FIG. 4 is a characteristic diagram showing the state of change in the current flowing through the electric circuit and the operating characteristics of the overcurrent detection device of the present invention, and FIG. 5 is the overcurrent detection device of FIG. 2. FIG. 6 is a waveform diagram showing how the capacitor is charged and discharged in FIG. In the figure, 10 is an AC line, 20 is a current transformer,
30 is a rectifier circuit, 40 is a burden circuit, 50 is a capacitor, 61 and 62 are resistors, 70 is a diode, 8
0 is an output device, 90 is a waveform conversion circuit, 100 is an A/D conversion circuit, and 110 is a microcomputer. Note that the same reference numerals in each figure indicate the same parts.

Claims (1)

[Scope of Claims] 1. Current detection and conversion means for detecting fault current flowing in an electric circuit and converting it into a digital signal; Level determination means comprising a microcomputer for determining the level of the digital signal; a time limit generating means comprising the microcomputer that performs a predetermined time limit operation corresponding to the level of the time limit generating means; an output means responsive to the time limit operation of the time limit generating means; thermal energy generated by fault current during the time limit operation of the time limit generating means; pulse generating means comprising the microcomputer, which generates a pulse having a pulse width corresponding to the pulse width at predetermined intervals during the time-limited operation; and a capacitor to be charged by the pulse, and and a resistor for discharging the electric charge in the form of a CR time constant circuit, and the residual voltage of the capacitor is calculated by the time limit generation means when the microcomputer is restarted after being reset or when a fault current occurs again. An overcurrent detection device comprising: charging/discharging circuit means for inputting an initial value to the microcomputer. 2. The overcurrent according to claim 1, wherein the charging/discharging circuit means is driven by a pulse having a pulse width controlled by the microcomputer to generate a constant current to charge the capacitor. Detection device. 3. The current detection conversion means includes a current transformer, and the secondary output of the current transformer is connected to the level determination means, the time limit generation means, the output means, the pulse generation means, and the charge/discharge circuit. The overcurrent detection device according to claim 1 or 2, which is supplied as a power source for operating the means. 4. The overcurrent detection device according to claim 3, wherein the operating power source is supplied from the secondary side of the second current transformer.