DE19648905A1 - Switching circuit for monitoring three-phase mains safety - Google Patents

Abstract

A switching circuit for monitoring safety fall-off or asymmetry in a three-phase mains supply compares the potential at an artificial starpoint (STP) formed through the resistors (R4,R5,R6) after the safety fuses (F1,F2,F3) with that of the mid point conductor (MP). If fuses are functioning and there is little or no asymmetry, the potentials will be virtually identical. However the circuit will operate if either one or even two phases are misfunctioning or there is asymmetry for any other reason, when the voltage difference (UD) overrides a predetermined threshold (2D1) to control a relay (RL1), supplied through the three phase rectifier diodes (D1,D2,D3). No auxiliary supply is needed and the circuit draws only a tiny current.

Description

The invention relates to a circuit arrangement for monitoring a safety device failure in the three-phase network.

Circuit monitoring is required to monitor a fuse failure in the three-phase network regulations known in which the voltage potentials before and after the fuses tials can be tapped via resistors to create an artificial star point to form. Between the two star points is the one of a Siche failure voltage occurring evaluated (see for example DE 41 18 346 C1).

Circuit arrangements for tapping the potentials are also only known after the 3 phase locks. It is also about resistance of the artificial star point after the fuses and its potential with that of the in this case also to be connected - the center conductor. This measuring principle can also be used to report the phase asymmetry.

The circuit arrangements of the monitoring devices usually contain one Output relay, the contact of which reports the fault status. It makes a difference you do that "Quiescent current principle" (output relay is energized in the "good state") and that "Open-circuit current principle" (output relay is energized in the event of an "error"). Alternatively, an optocoupler can be used as a signaling element instead of the relay.

The monitoring method with a neutral conductor connection is included in the variants and known without additional auxiliary voltage. In the first variant, the Supply of the internal electronics of the monitoring device, the output re relay and its control, an additional auxiliary voltage is required, either taken from the network to be monitored or created externally. In the the second variant is the operating energy for the monitoring device from the phase conductors to be monitored after the fuses.

The invention describes a circuit arrangement for fuse monitoring devices based on the second method (tapping the 3 phase potentials according to the fuses and center conductor MP) with the additional requirements "Open-circuit principle" and "without auxiliary voltage". The circuit described order is also suitable for monitoring for network asymmetry. "Open-circuit principle" means that the output relay of the monitoring device if one or two phase fuses or line asymmetry fail should pull.

"Without auxiliary voltage" means that in addition to the 3 phases to be monitored conclusions (after the fuses) and the center conductor of the three-phase network no further auxiliary voltage connections are provided for operating the device are.

It follows from both requirements that an - arbitrary - phase voltage already exists against MP must be able to the output relay of the monitoring device to get dressed. If all 3 phase voltages fail, no message should appear. H. no tightening of the output relay.  

A circuit arrangement known to solve this problem forms the artificial star point from the 3 phases with relatively low-resistance tapping conditions, so that in the event of a failure of the fuse then occurring Diffe limit voltage between this low-resistance artificial star point and the MP an output relay can be energized directly in the network.

The tapping resistors for the formation of the artificial star point must be so be low-impedance, that the differential voltage even in the event of failure of 2 phase chips voltage and permissible undervoltage in the upcoming phase can drive enough current to pull the output relay.

This circuit arrangement described lastly has the following disadvantages:

• 1) The "tapping resistances" for the artificial star point must be understandable be quite low impedance, so that in the event of a fuse failure, also with Mains undervoltage, a relay can be pulled. With that, also in the normal state with intact phase fuses, from the tapping devices a considerable heat dissipation is converted into heat, which is a multiple the operating power of the output relay. This prevents installation the monitoring device in small, space-saving housing.
• 2) Due to the already critical power balance, this circuit can be used arrangement practically only a total failure of one or two phases (phase voltage towards zero). However, in the event of a fuse failure in three-phase networks quite the case that - z. B. by larger, only partially loaded engines that continue on two phases - the third, failed phase voltage generato risch regenerated (fed back), and this u. U. with a height of ≧ 80% the mains voltage. The latter circuit arrangement can be such an unsym Do not recognize metrics in the three-phase network, because then the differential voltage between artificial star point and MP as well as the electrical power available there by far not sufficient to control an output relay.

The object of the invention was therefore to develop a circuit arrangement for fuse monitoring in the three-phase network, which works without additional auxiliary voltage, can also be used in the "open-circuit principle" and
firstly, has a low power consumption,
secondly, if the output relay picks up if up to two phases fail,
thirdly, it also reacts optionally with lower network asymmetry, as occurs in the event of a fuse failure due to regenerative regeneration of running motors.

According to the invention this object is achieved in that

• a) the circuit arrangement the generation of a permanent internal operating (Supply) voltage (for the output relay and its control) a 3-phase rectifier circuit provides, which this internal supply voltage, regardless of the good or fault condition of the network, both with 3, 2 or only 1 existing phase voltage is provided at a sufficient level, however in all cases with low power consumption;  
• b) the fuse failure / asymmetry by means of a separate, high-resistance formed artificial star point against MP is detected and
• c) ver the asymmetry signal thus obtained by means of an electronic circuit is strengthened and so, when a selectable threshold is reached, the output relay controls.

Explanations:
To a): In the invention, the fact is exploited that the 3-phase rectifier circuit on the downstream backup or filter capacitor always produces the same output voltage at a relatively low load, regardless of whether 3, 2 or only 1 phase feed. This ensures the supply of the output relay of the device and its control circuit in all cases to be considered.

Since the supply voltage generated internally by the rectifier circuit at "Open-circuit principle" of the output relay in the normal case (good condition of the fuse power or the network) is practically not charged, the power consumption of the Monitoring device negligible in this case. But also in the case of Fuse / mains failure when the output relay is energized is the lei Power consumption low, since the relay and the control circuit to the through the Rectifier circuit generated supply voltage can be adjusted and losses in series resistors thus remain low.

To b): Another advantage of the invention is that the fuse failure or Asymmetry over a high-resistance artificial star point against MP is detected, which also keeps the power consumption low.

Regarding c): Finally, the added benefit of this invention is that it is obtained in this way Asymmetry signal is electronically amplified, and thus depending on the design of the low response asymmetry to the on control of the output relay.

The invented circuit arrangement can thus be used not only as a fuse monitor, can also be used in networks with possible "feedback", but also find use as asymmetry monitor.

The 3-phase (or 3-pulse) rectifier circuit according to the invention can also be advantageously modified so that it works with "capacitive series resistors". As a result, no high-impedance, expensive relays and no electronic control components with a high voltage rating have to be used to adapt the circuit to the nominal voltage at higher nominal voltages, but inexpensive, low-impedance relays and components can be used without the power loss of the device both in the Idle state as well as when the output relay is activated. (This variant is treated in Fig. 2).

Likewise, according to the invention, the electronic amplification and evaluation of the asymmetry signal and the activation of the output relay can be implemented in different ways. It can e.g. B. a switching amplifier with discrete transistors can be used (see also FIG. 1) or one with an integrated comparator and optionally with a time delay stage ( FIG. 2).

2 embodiments of the invention are shown in the drawing and are explained in more detail below:

Fig. 1 shows a particularly cost-effective variant of the invented circuit arrangement of the monitoring device (S1) with a simple 3-phase rectifier circuit and evaluation of the asymmetry signal by means of a switching amplifier made of 2 transistors.

Fig. 2 shows a somewhat more complex circuit variant (S2) with "capacitive series resistors" in the 3-phase rectifier and evaluation of the Asym metry signal by means of a comparator and a time delay stage.

In Fig. 1, the 3 phase conductors of the three-phase network are designated L1, L2, L3, the associated center (neutral) conductor with MP. The phase conductors are usually protected (fuses F1, F2, F3) and continue to consumers, not shown. The potentials of the phase conductors after the fuses and the MP are connected to the monitoring device S1 (framed with dashed lines) with the associated terminals L1 *, L2 *, L3 *, MP *.

A 3-phase rectifier circuit is connected to the monitoring device S1 Terminals L1 *, L2 *, L3 *, MP * connected, each consisting of a diode D1, D2, D3 and associated series resistor R1, R2, R3. The cathodes of the diodes D1, D2, D3 lead to a common point, which is the positive pole of the internal Power supply + UB forms and to which the support capacitor C1 is connected is. C1 with its other connection to MP *, the negative pole of the internal one Power supply, connected. In parallel with C1 is a high resistance R10 switched on. A long lead leads from the positive pole of the internal power supply + UB rer current path via emitter - collector of PNP transistor T2 through the output relay RL1 to negative pole MP *. RL1 actuates an unsigned contact that reports the response of the monitoring device to the outside. Parallel to Relay RL1 is a free-wheeling diode D4. There is still a current from the positive pole + UB path through resistors R8 and R9 and the collector-emitter path from NPN-Tran sistor T1 led to negative pole MP *, the connection point from R8 to R9 is connected to the base terminal of T2. Parallel to the collector-emitter Line T1 is a capacitor C2, the function of which will be described.

At the terminals L1 *, L2 *, L3 * of the monitoring device are also one end of the resistors R4, R5, R6 connected to each other with their end form a common artificial star point STP. From that point leads a current path via Zener diode ZD1 on the basis of T1, the control the base - emitter of T1 extend the resistor R7 and the capacitor C3 are connected in parallel.

The functioning of the circuit arrangement according to FIG. 1 is as follows:
If the fuses F1 to F3 have not tripped, the potential of the artificial star point STP formed by R4 to R6 is almost identical to that of the center conductor MP * in the normal three-phase network (low asymmetries), so that no appreciable differential voltage UD occurs between them. As a result, the base of transistor T1 is not driven because of the threshold formed by ZD1. (R7 and C3 serve to suppress any interference). This means that no current flows in the collector circuit of T1; thus T2 and the output relay RL1 are not activated, C2 is charged to UB. The support capacitor C1 for the internal power supply of the relay and the control practically charges up to the peak value of the 3 phase voltages at L1 to L3. A persistent "overloading" of C1 due to interference peaks in the network is prevented by the high-resistance parallel resistor R10.

Triggers one or two of the fuses F1 to F3 or other green the greater network asymmetry, then exceeds that on the artificial star point STP against MP * applied differential voltage UD as specified by ZD1 given threshold, so that transistor T1 during the positive halfwaves of UD switches through, C2 discharges and controls T2 and relay RL1. By the Capacitor C2 is the collector-emitter voltage of T1 also in the negative Half waves kept low, since C2 charges relatively slowly via R8 / R9 can and is always discharged again in the positive half-waves. Through this "Bridging" is also for transistor T2 in the negative half-waves of UD Continuous basic control is guaranteed, relay RL1 picks up.

Even if 2 of the 3 fuses (or 2 of the 3 phase voltages L1 to L3) fail len, supplies the remaining supplying rectifier string of the 3-phase DC Richters R1 to R3 / D1 to D3 still have enough voltage at + UB so that the Supply of the control circuit and the output relay is guaranteed.

In Fig. 2, only the modified circuit arrangement of the supervising device itself (S2) is shown; the application in the three-phase network is the same as in FIG. 1. In S2, the three-phase rectifier circuit with D1 to D3 / R1 to R3, as already known from FIG. 1, is modified as follows:

Between the respective rectifier diode D1 to D3 and the associated series resistor stood R1 to R3 of each rectifier string, a capacitor C4 to C6 is on adds. From the respective connection points of the capacitors C4 to C6 to the Diodes D1 to D3 each carry a current path via a diode or Zener diode ZD2 to ZD4 for connection MP *.

Likewise, the circuit part for evaluating the asymmetry signal (UD between the artificial star point STP and MP * formed from R4 to R6) is changed as follows:
A diode D5 leads from the artificial star point STP to the + input of the integrated comparator K1. Also from + input K1, R7 and C3 are also switched to MP *. At the other (-) input of K1 there is a threshold voltage S which is derived from the internal operating voltage + UB obtained by the 3-phase rectifier circuit (not shown). The output of K1 is connected to the input of a time delay stage TV. Its output as in turn leads via resistor R11 to the base of transistor T3, which drives the output relay RL1. K1 and TV are supplied from the internal operating voltage + UB.

The 3-phase rectifier circuit in FIG. 2 functions similarly to that in FIG. 1, only here additional “capacitive series resistors” C4 to C6 are contained in the 3 rectifier strings. During the positive half-wave of a phase voltage against MP *, e.g. B. L1 *, flows via R1, C4 and D1, a charging current in the support capacitor C1 of the internal operating voltage supply + UB. This also charges C4. In the negative half wave of L1 * D1 blocks; however, the capacitor C4 can discharge again via ZD2 / R1, so that charging current can flow again via C4 in C1 (+ UB) during the following positive half-wave. The Zener diodes ZD2 to ZD4 limit the value of the rectified voltage + UB to the desired level for the output relay and the other electronics.

This modified rectifier circuit with "capacitive series resistors" has the advantage over that shown in Fig. 1 that you can generate a relatively small voltage + UB (z. B. 15 V), from which both a low-resistance, preisgün term relay and the integrated circuits for K1 and TV can be supplied directly without a large power loss in the series resistors of the rectifier circuit, which would be the case especially at higher nominal voltages. Zener diodes ZD2 to ZD4 can be replaced by normal diodes if + UB constantly has to drive a sufficiently large load current. Then a limitation of + UB is not necessary, since the load itself determines the height of + UB.

In FIG. 2, the asymmetry signal UD is first rectified by diode D5, sieved with C3 / R7 and compared with the threshold value S at the input by the + input of comparator K1. If the rectified asymmetry signal at the + input exceeds the predefined threshold value S, the output of the comparator K1 becomes high and controls the time stage TV. After the time step has elapsed, its output also goes high, controls transistor T3 via R11 and thus causes the output relay RL1 to pick up.

Common to both versions of the invented circuit arrangement (FIGS . 1 and 2) is the principle of the 3-phase rectifier circuit, which provides the internal operating or supply voltage + UB for the output relay and its control electronics, regardless of whether 3, 2 or only 1 any phase voltage are available.

Of course, the 3-phase rectifier circuit can in principle also be used with reversed polarity: Then D1 to D3 and in Fig. 2 also ZD2 to ZD4 are polarized the other way round, UB is negative against MP and the control circuit is modified accordingly.

Claims (8)

1. Circuit arrangement for monitoring a fuse failure or the asymmetry of a three-phase network by means of an artificial star point formed after the fuses, the potential of which is compared with that of the associated center conductor in order to generate a failure message therefrom by means of an evaluation circuit, characterized in that for the power supply a 3-phase rectifier circuit is used for the evaluation circuit and the relay or optocoupler reporting the failure / asymmetry, which is connected to the 3 phase conductors to be monitored and the center conductor of the network. 2. Circuit arrangement according to claim 1, characterized in that that each of the 3 strands of the 3-phase rectifier circuit is at least off a rectifier diode (D1 to D3), the diodes in a common feed the same point at which the internal supply voltage against MP - for the evaluation circuit and the relay reporting the failure (RL1) or the Optocoupler - is formed. 3. Circuit arrangement according to claim 1 or 2, characterized in that in each of the 3 strings of the rectifier circuit except the diode (D1 to D3) a series resistor (R1 to R3) is inserted. 4. Circuit arrangement according to one of the preceding claims, characterized ge features that in each of the 3 strings of the rectifier circuit an additional capacitor (C4 to C6) is inserted, and that of the rectifier diodes (D1 to D3) facing connections of the capacitors each a diode or Zener diode (ZD2 to ZD4) is switched to MP. 5. Circuit arrangement according to one of the preceding claims, characterized ge features that to form the artificial star point (STP), whose potential with the of the MP is compared, high-resistance resistors (R4 to R6) of the 3 Pha Connect the sensor connections to the common star point (STP). 6. Circuit arrangement according to claim 5, characterized in that the differential voltage UD between the artificial star point (STP) and MP is evaluated with a transistor switching amplifier, which is an output relay (RL1) or an optocoupler, the switching amplifier and the Re relay or the optocoupler from - through the 3-phase rectifier circuit generated - internal supply voltage is operated. 7. Circuit arrangement according to claim 6, characterized in that the switching amplifier additionally has a threshold through a Zener diode (ZD1) is specified. 8. Circuit arrangement according to claim 5, characterized that the differential voltage UD between the artificial star point (STP) and MP is evaluated with an integrated comparator circuit (K1), the Comparator and the relay or the optocoupler from the - through the 3-phase Rectifier circuit generated - internal supply voltage operated becomes.