JPS5837769B2 - overcurrent relay - Google Patents

Description

[Detailed Description of the Invention] The present invention detects the instantaneous maximum current of the secondary current of each current transformer connected to a multiphase AC circuit, and detects the maximum current at each instant when the maximum current exceeds a predetermined value. This invention relates to an overcurrent relay that operates a time-limited circuit or an instantaneous circuit to excite a trip coil and interrupt the circuit.

When protecting a polyphase AC circuit, the maximum current at each moment is detected from among the phase currents.
Overcurrent relays that are protected by one relay are advantageous.

In other words, in such a configuration, the output of the current transformer installed in each phase is extracted as a current, so there is no loss in the voltage converter, and the current transformer's output is This is because the load can be reduced, and since only the maximum current at each moment is output, the loss of the relay as a whole can be reduced.

In this type of overcurrent relay, voltage conversion is performed when the maximum detected current is applied as an input to a time-limited circuit or an instantaneous circuit, but in conventional overcurrent relays, this voltage conversion was performed directly using a resistor. .

In other words, the detected maximum current is supplied to a series circuit consisting of a resistor and a constant voltage circuit, the voltage obtained by the resistor is input to a time limit circuit or an instantaneous circuit, and the constant voltage obtained by the constant voltage circuit is input to a time limit circuit, etc. It is used as a control voltage.

However, since the maximum current is supplied from the secondary winding of the current transformer, in the conventional overcurrent relay described above, if the maximum current is ■, the resistance is R, and the constant voltage is E, the current transformer The burden on the current transformer is expressed as I (RI+E), and the burden IE caused by the constant voltage circuit has become an extra burden on the current transformer.

Further, in a large current region, the load on the current transformer increases in proportion to the square of the maximum current (2), so there is a problem that the load on the current transformer in the large current region becomes extremely large.

Therefore, in order to construct an overcurrent relay with a wide operating current range, it is necessary to use a current transformer with a large rating.

In view of the above, the present invention provides an overcurrent relay that does not place a large burden on the current transformer even in a large current region, and has excellent characteristics in terms of input voltage characteristics of instantaneous circuits and setting of operation level of time-limited circuits. The purpose is to

According to the invention, this purpose is achieved by introducing the detected maximum current into a constant voltage circuit, converting it into a voltage proportional to the maximum current by means of a resistor in the constant voltage circuit, and using this voltage as an input to a time-limiting circuit. be done.

In other words, with this configuration, the load on the current transformer becomes a constant voltage load, so the load on the current transformer in a large current region can be reduced, and the burden on the control voltage of time-limiting circuits, etc. can be eliminated. .

Furthermore, by configuring this constant voltage circuit so that it reaches saturation in extreme overcurrent conditions and the voltage between the terminals increases in proportion to the current, it can be easily detected as an input voltage of an instantaneous circuit and has excellent stable characteristics. voltage can be obtained.

Next, the present invention will be explained in detail based on embodiments shown in the drawings.

FIG. 1 shows a block diagram of an embodiment of an overcurrent relay according to the invention.

In the figure, 1 is the maximum current detection circuit, 2 is the pickup circuit, 3 is the constant voltage circuit, 4 is the time limit circuit, 5 is the instantaneous circuit, and R
is a resistance, and TC is a trip coil.

FIG. 2 shows a specific embodiment of the circuit shown in FIG. 1, in which the same components as in FIG. 1 are given the same reference numerals.

In such a configuration, the maximum current detection circuit 1 first detects current transformers CT, CT2 inserted into the three-phase AC circuits R, S, and T.
, CT3, the maximum current at each instant is detected, and the output current Ip resulting from this maximum current flows only through the pickup circuit 2 when it is below a predetermined value.

When the output current Ip increases and the voltage drop in the pickup circuit 2 exceeds the set value in the constant voltage circuit 3, the constant voltage circuit 3 operates and the output current Ip is divided into the pickup circuit 2 and the constant voltage circuit 3.

When the constant voltage circuit 3 operates and a part of the output current Ip is shunted to the Zener diode ZD1 in the constant voltage circuit 3, this current generates a voltage at the connection point between the resistors R2 and R3, and the base voltage of the transistor Tr is generated. A current flows and makes the transistor Tr conductive.

As the output current Ip increases, this transistor Tr is supplied with a larger base current, thereby causing a larger current to flow through the resistor R4, thereby maintaining the voltage between the terminals of the constant voltage circuit 3 at a substantially constant voltage value.

Since the conduction of the transistor Tr is thus controlled by the output current Ip, when the constant voltage circuit 3 starts operating, a voltage proportional to the output current Ip is generated between the terminals of the resistor R4.

When this voltage is applied to the time limit circuit 4 and such an overcurrent state continues for a predetermined period of time (this time depends on the input voltage of the time limit circuit 4), the time limit circuit 4 operates to reduce the risk Th. A signal is sent to the gate.

As a result, Cyrisk Th becomes conductive and trip coil T
Since the output current Ip is also shunted to C, the trip coil is excited, and the three-phase AC circuits R, S, and T are cut off all at once.

Further, when the output current Ip enters an overcurrent state of a predetermined value or more, the transistor Tr of the constant voltage circuit 3 becomes saturated, and a current of a predetermined value or more no longer flows through the resistor R4.

As a result, any further overcurrent will be handled by the Zener diode ZD.
1. Since it flows through a series circuit consisting of resistors R2 and R3, constant voltage circuit 3 has a fixed impedance, and resistor R2 +
Due to the voltage drop across R3, the voltage between the terminals of the constant voltage circuit 3 increases.

In this manner, when the output current Ip exceeds a predetermined value, the voltage between the terminals of the constant voltage circuit 3 increases in proportion to the output current Ip.

Therefore, only when the output current Ip reaches an extreme overcurrent state, a large voltage is applied to the instantaneous circuit 5, which operates instantaneously to send a signal to the gate of the cyrisk Th, and the above-mentioned trip coil TC is excited. 3-phase AC circuit R, S
, T are cut off all at once.

Note that although the resistor R3 is provided in the embodiment shown in FIG. 2, this resistor R3 is not provided and the voltage between the terminals of the pickup circuit 2 is applied between the base and emitter of the transistor via the Zener diode. A constant voltage circuit can be constructed even if the

The maximum current detection circuit 1 can be composed of, for example, three full-wave rectifier circuits Re1, Re2, and Re3 connected in series, and since this circuit is already known, its operation will not be explained in particular.

Pickup circuit 2 includes resistors R1 and R.

and a resistance R. Let be variable.

In addition, the constant voltage circuit 3 includes a Zener diode ZD1 and a resistor R.
2 to R4, and transistors Tr.

The pickup circuit 2 and the constant voltage circuit 3 are constructed in this way, and the resistance values R'1 and R'o of the resistors R1 and Ro are set when the current flowing through the three-phase AC lines R, S, and T is in a steady state. When the value of the output current Ip is I1)1, the settings should be made so that the following relationship holds: ■p1×(R'1+R<■ZD1 (where VZD is the Zener voltage of the Zener diode ZD1) Accordingly, 3
When the currents flowing through the phase AC circuits R, S, and T are in a steady state, the output current Ip flows only through the pickup circuit.

As a result, the current flowing through the three-phase AC circuits R, S, and T enters an overcurrent state, and the value of the output current Ip becomes ■p2, and Ip2 X (R'l + R'0) > VZD
If the relationship 1 is established, then the Zener diode Z
A current is supplied to the base of the transistor Tr via D1, and the transistor Tr becomes conductive.

Due to the conduction of the transistor Tr, the output current Ip is increased by the resistance R4.
It is also divided into

As mentioned above, for the output voltage iIp below a predetermined value, the transistor Tr does not saturate and the Zener diode ZD1
Since the conduction is controlled by the base current supplied through the resistor R4, the current flowing through the resistor R4 changes in proportion to the output current Ip.

The voltage across the terminals of this resistor R4 is applied to the time limit circuit 4 as an input.

The time limit circuit 4 can be composed of a resistor R5, a transistor Tr1, a capacitor C, and a switching element T.

The conduction of the transistor Tr1 is controlled by the voltage across the terminals of the resistor R4, and the capacitor C is charged.

When the charging voltage of the capacitor C reaches a predetermined value, the switching element T becomes conductive, and the discharge current of the capacitor C supplies an ignition pulse to the gate of the thyristor Th, ignites the thyristor Th, and excites the trip coil TC. can be done.

As the switching element T, for example, a two-terminal bidirectional thyristor SSS (silicon symmet
rical switch) can be used.

The instantaneous circuit 5 can consist, for example, of a Zener diode ZD2 and a resistor R5.

When the current flowing through the three-phase AC circuits R, S, and T reaches an extreme overcurrent state, the transistor Tr reaches saturation due to the corresponding output current Ip, so the voltage between the terminals of the constant voltage circuit 3 is reduced by the current flowing through the resistor R4. increases in proportion to

Therefore, by configuring the instantaneous circuit 5 using the Zener diode ZD2, the instantaneous circuit 5 operates when the voltage between the terminals of the resistor R4 exceeds the Zener voltage of the Zener diode ZD, and the trip coil TC is activated as described above. Excitation is performed.

In the overcurrent relay according to the present invention shown in FIG. 1 or 2, by using the pickup circuit 2 and the constant voltage circuit 3, the following effects that could not be obtained with conventional devices can be obtained. .

First, the resistor R in the pickup circuit 2.

By making variable, the operating level of the constant voltage circuit 3,
That is, the minimum value of the current flowing through the three-phase AC circuits R, S, and T necessary for operating the time limit circuit 4 can be easily and stably set.

Each circuit is connected to the current transformer as a load, and the negative current of the current transformer is expressed as the product of current and voltage (VA), and in the present invention, by using the constant voltage circuit 3, the current When the current transformer becomes larger, the impedance is reduced and the voltage becomes constant, so the load on the current transformer is determined by the product of the output current Ip and the predetermined voltage value in the constant voltage circuit 3 (constant voltage circuit 3
Instantaneous circuit 5 is used for overcurrent that saturates the transistor.
operates to perform instantaneous cut-off), and the load on the current transformer in the overcurrent region can be reduced compared to conventional devices.

Furthermore, by using the constant voltage circuit 3 to provide a constant voltage load, the resistance equivalently decreases as the current increases, so that saturation of the current transformer can be compensated for.

By combining the pickup circuit 2 and the constant voltage circuit 3, the pickup circuit 2 can be implemented as a simple configuration including only a resistor.

In addition, when obtaining the excitation current to the trip coil TC from the secondary current of the current transformer (generally, the excitation current of the trip coil TC is obtained from a separate DC power supply), the constant voltage circuit 3 is disconnected due to conduction of the sirisk Th. In operation, most of the output current Ip can be supplied to the trip coil TC so that a sufficient excitation current for the trip coil TC can be obtained.

In other words, the conduction of the thyristor Th brings the circuit of FIG. 2 into a state equivalent to the equivalent resistance of the trip coil TC being connected in parallel.

Therefore, the values of the resistors R1 and Ro and the selection of the trip coil TC are set so that the voltage drop due to the current flowing through the resistors R1 and Ro and the trip coil TC is smaller than the Zener voltage value of the Zener diode ZD1 of the constant voltage circuit 3. As a result, when the trip coil TC is excited, the constant voltage circuit 3 is made inactive so that a sufficient excitation current for the trip coil TC can be obtained.

Further, according to the present invention, without providing the pickup circuit 2,
The operation level may be set in the time limit circuit 4.

That is, in a configuration in which the pickup circuit 2 is not provided, the constant voltage circuit 3 is operated by the minute output current Ip of the maximum current detection circuit 1, and a voltage corresponding to the output current Ip is obtained at the resistor R4. What is necessary is to set the operation level at .

Even in such a structure, a sufficient excitation current for the trip coil TC can be obtained by making sure that the voltage drop in the trip coil TC is equal to or lower than the operating level of the constant voltage circuit 3 when the trip coil TC is excited.

Note that the circuit shown in FIG. 2 is merely an example, and it goes without saying that the overcurrent relay according to the present invention is not limited to this.

As described above, the overcurrent relay according to the present invention not only solves the problems of conventional overcurrent relays, but also has characteristics not found in the past, and has many effects in protecting circuits from overcurrent. 0 that is a thing

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an overcurrent relay according to the present invention, and FIG. 2 is a specific circuit diagram of the overcurrent relay shown in FIG. 1...Maximum current detection circuit, 2...Pickup circuit, 3...Constant voltage circuit, 4...
...Time-limited circuit, 5...Momentary circuit, TC...
...Trip coil.

Claims (1)

[Scope of Claims] 1. A maximum current detection circuit whose output current is the maximum current at each moment among the secondary currents of each current transformer connected to a multiphase AC circuit, and an output terminal of the maximum current detection circuit. A series circuit consisting of a trip coil and a switching element and a pickup circuit are connected in parallel between each other, and a voltage corresponding to the voltage between the terminals of the pickup circuit is connected to the base terminal through a series circuit consisting of a Zener diode and a first resistor. A constant voltage circuit consisting of a transistor whose emitter and collector are connected in parallel to the pickup circuit via a second resistor, the voltage across the terminals of the second resistor is applied, and the a time limit circuit that supplies a gate signal to the switching element after a time limit; a voltage between the terminals of the pickup circuit is applied; and when the voltage between the terminals exceeds a predetermined value, the gate signal is instantaneously supplied to the switching element; An overcurrent relay characterized by consisting of an instantaneous circuit. 2. The overcurrent relay according to claim 1, wherein the pickup circuit is configured to include a variable resistor.