NL192415C - Protection system for long, relatively light-duty high-voltage power lines or cables. - Google Patents

Description

1 192415

Protection system for long, relatively light-duty high-voltage power lines or cables

The invention relates to a protection system for long, relatively lightly loaded high-voltage power lines or cables. In practice, the following problem arises: 5 When protecting a long, relatively lightly loaded high-current line or cable, two parameters are important, namely the voltage losses and the short-circuit power.

With increasing impedance, in the event of a short circuit, the switch-off time becomes longer.

When designing power lines one has to take into account a number of factors: - the heating of the conductors as a result of the current passage, both during normal operation and short circuit; - the voltage losses between the supply point and the connected appliance; - the tripping time of the protection device in case of overload and defects.

At almost most installations, these requirements appear to meet in a reasonable manner.

However, when it comes to long, lightly loaded lines such as these occur in the power supply of eg road lighting installations, the following appears to be acceptable conductor cross section.

b) During normal operation, the current load is low compared to the maximum permissible value for this conductor, so that the conductor temperature remains clearly below the permissible highest value.

C) The conductor cross-section calculated for the voltage losses only meets the requirements for short-circuit power over a limited length.

With increasing cable length, the minimum required short-circuit current is no longer reached, see figure 1.

The consequence of this is that the switch-off time of the protection exceeds the maximum value that has been set for this in the "Safety provisions for Low-voltage installations NEN1010 - 4th edition". In order to comply with the set cut-off speed, it is necessary to choose according to larger pipe cross-sections using the conventional protection methods.

This can lead to economically less responsible structures.

The safety system currently proposed is aimed at achieving a sufficiently short switch-off time over the entire path of the cable in the event of defects, while maintaining the line cross-section resulting from voltage drop calculation.

According to the invention this is achieved with a combination of a thematic / magnetic and a delayed acting magnetic characteristic, wherein the thermal / magnetic part per phase consists of a circuit breaker, which functions completely independently with larger short-circuit currents (such as these can occur in the first part). of the line to be protected) and the added magnetic part, which per phase consists of a current measuring relay fed via a current transformer, which is energized when a set value is exceeded, whereby the activation of this current measuring relay interrupts the activation of the magnetic switch to the outgoing line .

According to a further elaboration of the inventive idea, the delay time to be set is selected in accordance with the maximum allowable touch time.

The current setting can be selected at a value which remains above the rated value of the load current, but below the lowest occurring short-circuit current and below the highest permissible current for the line.

The operation is such that the conditions of the protection are met at all points of the cable route, both with regard to the risk of contact and the protection of the cable.

45 The thermal part works on the first part of the route where sufficiently high short-circuit currents still occur, and further on the cable, the magnetic part takes over the protective function.

Thus, the present invention provides an additional feature that eliminates the need for a larger conductor cross section for the residual short-circuit capability. The protection system must be specifically adjusted for each design. The selectivity with regard to both the previous and the previous safeties is maintained.

The circuit is designed in such a way that in the event of faults in three-phase systems, only the defective conductor is switched off, see figure 2.

A renewed automatic turn-on pulse cannot cause a safety-disabled conductor to be energized again.

55 It is to be noted that from U.S. Pat. No. 3,636,410 is known an electrical breaker of three-phase mains in the event of a short circuit or sustained high currents in a mutually selective system, i.e. a system which also contains other protections.

192415 2

Furthermore, German "Auslegeschrift" 11.90.092 discloses an electrical overload switch, for example for a motor, in particular with heavy, slowly decreasing starting current with only thermal breaker and an added part that influences the switch-off time. The tripping characteristic, that is to say the current-time curve of such a thermal switch, follows from the current which influences the interrupter, from the forces which the switch must apply and the thermal time constant of the interrupter usually constructed as bimetal.

Finally, U.S. Pat. No. 4,090,156 describes multipolar switches with thermal-magnetic tripping in general, and more particularly circuit breakers equipped with semiconductor automatic trip units responsive to certain fault currents.

There are always two differences with respect to these specifications: - mast flow principle of the present invention versus work flow principle of the other publications; - phase protection against contact, with adjustable time limit.

The invention will be further elucidated on the basis of an exemplary embodiment shown in the drawing ........ -—------------

In the drawing: figure 1 shows the known course of voltage losses and short-circuit current as a function of the cable length; figure 2 shows the relationship between short-circuit current and switch-off time of the protection system according to the invention in relation to the cable length; figure 3 shows a current / time characteristic of a protection system according to the invention in relation to other electric values of interest; figure 4 shows a wiring diagram for a complete 3-phase group, carried out according to the invention; Figure 5 shows a circuit diagram for a complete 3-phase group according to the invention; Figure 6 shows a control flow diagram for this 3-phase group, according to the invention.

Figure 1 shows the voltage losses AU and the variation of the short-circuit current 1k as a function of the cable length under the schematically indicated cable. With increasing length, the minimum required short-circuit current 1k <min) is no longer reached and the area further away would essentially no longer be adequately protected in terms of tripping time. The switch-off time of the protection, as indicated in the introduction, will exceed the officially established maximum value.

Figure 2 shows the short-circuit current as a function of the cable length in the upper part, and the switch-off time in the lower part. The remote area where the minimum required short-circuit current is no longer reached is magnetically protected, the area in front is thermally protected.

Figure 3 shows the security characteristic of the present security system.

The tripping time is turned off vertically and the short-circuit current horizontally. The total operating current and the total inrush current are also shown. The protection characteristic of an end consumer connected to the cable is also indicated. It is essential that selectivity remains between the switch-off characteristic of the protected devices and the switch-off characteristic according to the invention (group-40 protection).

The circuit can be set up for 1 phase as well as for 3 phases, see figures 4,5 and 6.

The thermal part consists per phase of a circuit breaker which functions completely independently with larger short-circuit currents (such as these can occur in the first part of the line).

The magnetic part consists per phase of a 45 current measuring relay (M1, 2,3) fed via a current transformer (TR1, 2, 3) and energized when a set value is exceeded.

Activating this current measuring relay interrupts the energization of the contactor (A1, 2, 3) to the downstream line.

The delay time of the time relay (T1, 2, 3) is chosen according to the maximum allowable touch time.

50 The current setting is selected at a value that remains above the rated value of the circuit breaker, but below the lowest occurring short-circuit current and below the highest permissible current for the line.

The selectivity with regard to the underlying security means remains guaranteed, see figure 3.

Claims (3)

3 192415 Protection system for long, relatively lightly loaded high-voltage power lines or cables, characterized by a combination of a thermal / magnetic and a delayed-acting magnetic characteristic, wherein the thermal / magnetic part per phase consists of a circuit breaker, which functions completely independently at larger short-circuit currents (such as may occur in the first part of the line to be protected) and the added magnetic part, which per phase consists of a current measuring relay fed by a current transformer, which is energized when a set value is exceeded, whereby the response of this current measuring relay interrupts the actuation of the contactor to the 10 outgoing line. Protection system according to claim 1, characterized in that the delay time to be set is selected in accordance with the maximum permissible contact time. Protection system according to claim 1 and / or 2, characterized in that the current setting is selected at a value which remains above the nominal value of the load current, but below the lowest short-circuit current occurring and below the highest permissible current for the line. . Hereby 6 sheets drawing