NL8902169A - Circuit protection system for electrical distribution cables - protects long cables carrying relatively small normal currents by responding to heat and magnetic fields - Google Patents

Abstract

The system is designed to protect a 380 volt, star connected, three-phase distribution cable against short circuits between phases, between phase and neutral and between phase and earth. The system, known as `PRO-TECH-2000', is affected by two effects of a short circuit on the cable, namely, the thermal and magnetic effects. The value of the short circuit current on a long cable depends on the distance of the short from the protected current source. The system therefore has cut-outs which react to the thermal effect of high currents and cut-outs which react to the magnetic effect of smaller currents.

Description

Protection device PRO-TEC-2000.

Explanation of the principle and construction of a device for securing long, relatively lightly loaded high-voltage lines in an economically responsible manner.

The problem.

When designing power lines, a number of factors must be taken into account: - the heating of the conductors as a result of the current passage, both during normal operation and in the event of a short circuit.

- the voltage drop 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.

When it comes to long, lightly loaded pipes such as these occur in the power supply of, for example, road lighting installations, the following appears.

a) Based on a predetermined maximum permissible voltage drop, the calculation leads to an 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 drop 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 FIG 1.

The consequence of this is that the switch-off time of the protection exceeds the maximum values set for this in the "Safety regulations 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 safety c) methods.

This can lead to economically less responsible structures.

The combined security system PRQ-TEC-2000.

The protection system proposed by us 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 conductor cross-section resulting from voltage loss calculation.

This is achieved with a combination of a thermal and a delayed acting magnetic characteristic.

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

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.

The safety device must be specifically adjusted for each design.

The selectivity with regard to both the present and the underlying safeties is maintained.

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

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

Construction and operation.

The circuit can be set up for 1 phase as well as for 3 phases, see drawing 89-2000, page 1,2,3.

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 current relay supplied by a current transformer, which is energized when a set value is exceeded.

The activation of this current relay interrupts the activation of the contactor to the downstream line.

The delay time is selected in accordance with the maximum allowable touch time.

The current setting is selected at a value which 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 underlying security means is guaranteed, see FIG 3.

Claims (1)

CONCLUSION. The above describes a safety device for long, relatively lightly loaded high-voltage lines. Application becomes effective when the pipes become so long that the conventional protection means can no longer meet the requirements of the switch-off time. A patent is pending on the application and construction of the security device.