Power System Protection Presentation

1. Power System Protection Presentation
High Voltage Circuit Breakers: SF6 vs. Vacuum
Presented By:
Adeen Syed
2009-EE-391

2. Abstract
• The aim of this paper is to review the existing
different types of HV CB, comparing the most
commonly used, i.e. SF6 and Vacuum CB, which has
been developed and recently applied for HV, and
could become the future environmental friendly HV
CB.

3. Introduction
 Circuit Breaker & its Principle:
• A switching device which can open or close a circuit and is achieved by the
separable contacts and allows to interrupt the current.
CBs can be grouped into following:
• air magnetic circuit breakers.
• air blast circuit breakers.
• oil circuit breakers.
• sulphur hexafluoride (SF6) circuit
breakers.
• vacuum circuit breakers.

4. SF6 Circuit Breakers
• SF6 CBs replaced oil and air CBs for HV applications due to its high
dielectric strength.
• The current continues flowing after contact separation through an arc
plasma of ionized SF6. A constant gas flow is directed to the arc that
extracts heat from it and so allows its extinction.
• The gas flow deionizes also the
contact gap and re-establishes
the required dielectric strength
to prevent an arc restrike.

5. Gas Flow in SF6 CBs
 The direction of gas flow can either be:
• parallel.
• across the axis of the arc.
 Cross gas flow cooling of the are is achieved by making the arc move in
the stationary gas.
 Axial gas flow creates a turbulence
causing interaction b/w gas and plasma.
This needs, a differential pressure and is
created along the arc.
• Later, the arc energy was itself used to
create a differential pressure to reduce the
cost.

6. Vacuum CBs (VCB)
General Features:
•Contacts separate and current
flowing through electrodes initiate
a metal vapor arc discharge in the
contacts called the vacuum arc.
•It flows through the plasma. The
arc is then extinguished and the
conductive metal vapor condenses
on the metal surface.

7. Modes of Vacuum Arcs in VCBs
Different modes of vacuum arcs and depend on the current level and the
size of the contacts.
Diffuse Mode:
•This is assumed at lower currents. Here one or more cathode spots move
constantly over the contact surface. These spots repel each other and have
a finite and variable lifetime.
Constricted Mode:
•The arc changes in appearance with the changing current and the plasma
is focused on a small area of the electrode, about 1 cm2, so instead of
multiple moving cathode spots the appearance is a constricted mode.

8. Modes of Vacuum Arcs Contd.
• Transition from diffuse to constricted mode occurs between 10 and 15
kA depending on the size of the contact and the material . Constricted
mode has some disadvantages - the interruption of the current is difficult
and produces severe erosion on contacts.
• To overcome this the root must be
moved over the contact surface by the
appropriate shape of the contacts called
the “radial magnetic field or spiral
contacts” so that the current flowing
through them results in a radial magnetic
field. This rotate the arc root rapidly
around the contact resulting in a uniform
heat distribution over its surface.

9. Modes of Vacuum Arcs Contd.

10. MV Application in VCB
 Medium voltage is where VCB has its wide applications
ranging between 5 to 38 kV.
 Both the diffuse and constricted modes of arc is found
here and the switching of arc from one form to other is
done by subjecting it to radial or transverse magnetic
field.

11. HV Application in VCB
• At high voltage, the arc adopts a constricted form and arc quenching
is more difficult. As the size of the contacts will be large enough for
HV application to transition from diffused to constricted will not be
achieved by radial magnetic field.
• Hence for HV, axial magnetic
field through the axial magnetic
field electrodes is applied by
leading the arc current through a
coil arranged outside the
vacuum chamber.

12. Comparison b/w SF6 and VCB
SF6 CBS
•Use SF6 as a quenching media.
•Better dielectric strength.
•Reduce the size of electrical
equipment.
•SF6 is inert, nonflammable,
noncorrosive, odorless and non-toxic
gas.
•Labelled as one of the major global
warming gases.
•Less cost.
•Difficult to maintain.
VCB
•Quenching media is vacuum.
•Environment friendly.
•Simple construction and small
number of components.
•Long operation live in terms of high
number of operating cycles.
•Compact size and light weight.
•High dielectric resistance recovery
after zero current.
•No risk of explosion or fire.
•More cost.
•Easily maintenance.

13. Comparison of Dielectric Strengths

14. Conclusion
• Power demand is growing day by day so it is
important t develop new CB technology.
Compromise is must when it comes to the CB
selection.