Introduction

The LW36-126/T3150-40 outdoor high voltage SF6 self-blast circuit-breaker (third-generation SF6 circuit breaker) uses the unique self-blast arc extinguishing principle for current interruption, thus obtaining an outstanding performance in current interruption at all breaking current values. A major part of the energy needed for the arc interruption is taken from the arc itself, thus reducing the mechanical energy required from the operating mechanism, therefore allowing the mechanical life to reach a new level of 10,000 operations. The reliability of the circuit breaker is thoroughly improved by this principle.







Circuit breaker Structure

The following picture shows the structure of the circuit breaker, three poles are mounted on a common pole frame. The control cubicle holds the spring driving mechanism and the control units and is mounted at the center of the poleframe underneath the middle pole. The output rod of the mechanism connects with the crank of central phase.


Poles

Each pole is an individual gas-tight unit. As shown in the following picture, a pole is composed of upper terminal plate, interrupter, lower outlet block, porcelain bushing, insulating pushing rod, crankarm and operating shaft.


Terminals

Upper and lower terminal plates are used for primary connection. The lower plate can be connected both on the front and the rear side, (access to the control cubicle is on the front side, showing also the position indicator and density monitor in the pole frame).

Normally the upper terminal is mounted for front side connection. On customers request it can also be delivered for rear side connection. The drilling dimensions of the terminal plates are according to the recommandation for 3150A and 1250A in the standard GB5273 "Terminals of transformers, high-voltage apparatus and bushing". See figure 3


Design of the Interrupter Unit

The interrupter is enclosed in the porcelain insulator and is the core unit of the circuit breaker. It includes insulator, fixed main contact, fixed arcing contact, nozzle, cylinder, moving arcing contact, moving main contact, moving contact support, push rod etc. (see figure 4) Absorbent is mounted in the upper part of the fixed contact. The push rod links to the insulating rod in the bushing then to the drive shaft of crankarm case. Interrupter bushing is made from high-strength porcelain which has high strength and good gas-tightness.


Structure and operating principle of the circuit breaker


Principle of arc extinguishing

When the circuit breaker receives the opening signal, the moving contact assembly - cylinder, moving main and arcing contact, pull rod and other moving parts are moved downward by the opening spring forces. Firstly, the main contacts part and the current transfers to the inner arcing contacts are still closed. Then the arcing contacts separate leading to an arc between them.

On short-circuit interruption due to the high fault current, the arc zone gets a high energy. The thermal energy of the arc zone is transferred into the thermal expansion chamber creating a low-temperature high gas pressure in the chamber. As the pressure in the thermal expansion chamber gets higher than the pressure in the compression chamber the control valve closes.

When the arc current approaches current zero, high pressure gas will flow through the nozzle and extinguish the arc. The gas in the compression chamber will be compressed during the circuit breaker opening operation. However, at a certain pressure, the overpressure relief valve will open and allowing the gas to escape. Therefore the operating mechanism does not have to counteract higher gas pressure and less mechanical energy and force is needed.

When breaking low current (normally below several kilo amps), due to the low arcing energy, not sufficient gas pressure is generated in the thermal expansion chamber. In this case, the gas pressure of the compression chamber is higher than the pressure of expansion chamber, the control valve is open and the compressed gas will blow through the contacts. When the arc current reaches zero, the pressurized gas flows through the nozzle and extinguish the arc.


Foundation for Frame



Installation of pole frame and control cubicle



Key technical data of the product


Key technical data of the circuit breaker of LW36-126




Technical data of circuit-breaker components



Secondary schematic diagrams




Secondary schematic diagrams