Interrupter/Operator Description; Introduction; Vacuum Interrupters; Stored Energy Operating Mechanism - Siemens SDV-4A Instruction Manual

Interrupter/Operator Description

Introduction

Type SDV circuit breakers include three vacuum interrupt-
ers, a stored energy operating mechanism and necessary
electrical controls. On some circuit breaker ratings, insulat-
ing barriers are located between the vacuum interrupters.
This section describes the operation of each major subas-
sembly as an aid in the operation, installation, maintenance
and repair of the type SDV vacuum circuit breaker.

Vacuum Interrupters

The operating principle of the vacuum interrupter is simple.
Figure 9 is a cutaway view of a typical vacuum interrupter.
The entire assembly is sealed after a vacuum is established.
The interrupter stationary contact is rigidly attached to the
end cap which serves as one terminal of the interrupter.
The interrupter movable contact is free to move in a guide,
and is connected to the operating mechanism by a system
of linkages. The metal bellows assembly provides a se-
cure seal around the movable contact, preventing loss of
vacuum while permitting movement of the contact.
Fixed
Contact
Current
Connection
Ceramic
Insulator
Arc Shield
Fixed
Contact
Moving
Contact
Ceramic
Insulator
Metal
Bellows
Guide
Moving
Contact
Current
Connection
Figure 9. Cutaway View of SDV Vacuum Interrupter.
When the two contacts separate, an arc is initiated which
continues conducting up to the following current zero. At
current zero, the arc extinguishes and any conductive metal
vapor which has been created by and supported the arc
condenses on the contacts and on the surrounding vapor
shield. Contact materials and configuration are optimized
to achieve arc motion and to minimize switching distur-
bances.
The arc drawn in the vacuum breaker is not cooled. The
metal vapor plasma is highly conductive and the resulting
arc voltage only attains values between 20V and 200V. For
this reason and because of the short arcing times, the arc
energy developed in the breaker is very small. This also
accounts for the long life expectancy of the vacuum inter-
rupter.
Phase barriers are provided on all 25.8kV and 27.6kV class
type SDV units as shown in Figure 10. These plates of in-
sulating material are attached to the circuit breaker hous-
ing and provide suitable electrical insulation between the
vacuum interrupter primary circuits. Phase barriers are not
required in 15.5kV class type SDV units, but are available
as an option.
Figure 10. Breaker with Interphase Barriers.

Stored Energy Operating Mechanism

The stored energy operating mechanism of the SDV circuit
breaker is an integrated arrangement of springs, solenoids
and mechanical devices designed to provide a number of
critical functions. The energy necessary to close and open
the contacts of the vacuum interrupters is stored in power-
ful tripping and closing strings. These springs are normally
charged automatically, but there are provisions for manual
charging. The operating mechanism that controls charg-
ing, closing and tripping functions is fully trip-free, i.e.,
spring charging does not automatically change the posi-
tion of the primary contacts, and the closing function may
be overridden by the tripping function at any time.
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