High Voltage Switchgear
5.4 Medium Voltage Circuit Breakers
Chapter 5
On-board ships the switchboard is usually subdivided. The subdivision may be effected by the bus tie circuit breakers or other suitable means, with the generator inputs and duplicated services split equally.
High Voltage Switchgear
The dielectric strength of the media in between the contacts can be increased in numerous ways such as:
a) Compressing the ionized arcing media; compressing accelerates the de-ionization process of the media,
b) Cooling the arcing media; cooling increases the resistance of the arcing path c) Replacing the ionized arcing media by fresh gasses.
In marine applications, medium voltage (3.3 to 11 KV) switchgears are used and according to their arc-quenching media, the most commonly used circuit breakers are:
d) The vacuum circuit breaker.
e) The SF6 circuit breaker.
5.4.1 The Vacuum Interrupter
Chapter 5
It is mainly suitable for medium voltage switchgear where the arc-quenching takes place in vacuum. The operation of the breaker (opening and closing of the current-carrying contacts and the associated arc interruption) takes place in a vacuum chamber within the breaker, known as the vacuum interrupter. Vacuum offers the highest insulating strength. So, it has far superior arc quenching properties than any other medium. For example, when contacts of a breaker are opened in vacuum, the interruption occurs at the first current zero with dielectric strength between the contacts building up at a rate that is thousands of times higher than that obtained with other circuit breakers.
5.4.1.1 Advantages of a Vacuum Circuit Breaker
Fixed Contact Stem
Fixed Terminal Pad
Ceramic Insulator
Fixed Contact Metal Shield Moving Contact
Insulator
Bellows Bearing
Mechanical Coupling for Operating Mechanism
Moving Contact Stem
The service life of a vacuum circuit breaker is much longer than other types of circuit breakers. There is no chance of a fire hazard as in the case of an oil circuit breaker.
It is also environment-friendly unlike the SF6
circuit breaker that poses a risk of gas leaks.
Replacement of the vacuum interrupter is also convenient. The vacuum circuit breaker is the most reliable current interrupter for medium voltage switchgear; it requires minimum maintenance as compared to other circuit breaker technologies.
The interrupting chambers are made of porcelain and are sealed. They cannot be opened for maintenance; however, the operational life is expected to be about 30 years, if the vacuum is maintained. Since the dielectric strength of vacuum is high, the interrupters are small. The gap between the contacts is about 1 cm for 15 kV interrupters and 2 mm for 3 kV interrupters.
Figure 5.14
The Vacuum Circuit Breaker
High Voltage Switchgear
5.4.1.2 Construction of a Vacuum Interrupter 5.4.1.2.1 Breaker Contacts
Some of the alloys used as vacuum circuit breaker contacts are copper-bismuth, silver- bismuth, silver-lead and copper-lead. Usually. the contacts have large disc-shaped faces with a large stem. The disc faces contain spiral segments so that, the arc current produces axial magnetic fields. Such a geometry causes the plasma of the arc to move rapidly over the contact surface and hence minimizes the metal evaporation and erosion of the contacts due to arc.
5.4.1.2.2 Vapour Condensing Shield or Metallic Shield or Spotter Shield
These are supported on an insulated housing and are placed in between the contacts and the enclosure such that they enclose the contact region. It prevents the metal vapour (released during arcing) from reaching the enclosure and condensing on it. So, condensing of metal vapour takes place on these shields only. Usually, this shield is made of stainless steel.
5.4.1.2.3 Metallic Bellows
The two ends of metallic bellows of vacuum circuit breaker are welded to the lower end flange and the moving contact. They are used to permit the movement of the contact inside the sealed construction. They are usually made of stainless steel. To perform satisfactory repeated operations, the design of the bellows plays a significant role.
5.4.1.2.4 End Flanges
The end flanges of the vacuum circuit breaker support the fixed contact. sputter shield, outer insulating enclosure, metallic bellows and their protective shield. They are made of non- magnetic metal.
5.4.1.2.5 Enclosure or Outer Envelope
Generally, it is made up of glass because glass is a non-porous, impermeable insulating material which can retain high vacuum. Also, it is quite easy to join it to the end flanges.
Chapter 5
5.4.1.3 Operation of a Vacuum Interrupter
The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas, which makes it possible to quench a vacuum arc within a very small contact gap. The main function of any circuit breaker is to quench an arc during a current-zero crossing, by establishing high dielectric strength in between the contacts so that the re-establishment of the arc after a current-zero condition becomes impossible.
The drive energy required in a vacuum circuit breaker is minimal due to its short contact gap, low contact mass and no compression of the medium. When two face-to-face contact areas are just being separated from each other, they are not separated instantly; the contact area on the contact face is progressively reduced and ultimately comes to a point where they are finally separated. At the instant of separating of the contacts in a medium of vacuum, the current through the contacts is concentrated on that last contact point of the contact surface and it results in a hot spot. Since the medium is vacuum, the metal on the contact surface is easily vapourised due to that hot spot and it creates a conducting media for the arc path. Then the arc will be initiated and continued until the next current zero. At current zero, this vacuum arc is extinguished and the conducting metal vapour is re-condensed on the contact surface.
Figure 5.15 Contrate*
(Segmented Contacts)
Figure 5.16
Spinning of the Arc due to Electromagnetic
Forces
Figure 5.17 Spiral Contacts
* Having cogs or teeth projecting parallel to the axis (at right angles to the plane of rotation) instead of radiating from it.
High Voltage Switchgear
The re-establishment of arcs is prevented by producing a high dielectric strength in the contact gap after a current zero condition.
CuCr is the ideal material used for current-carrying contacts; it plays an important role in the performance of the vacuum circuit breaker. The contact geometry has gradually changed to a spiral-shape, cup-shape and axial magnetic field contact from the earlier butt contact.
Figure 5.18
Geometry of a Radial Magnetic Field Contact with a Rotating Vacuum Arc 5.4.1.3.1 Interruption Process in a Spiral Contact
a) To prevent overheating and contact erosion, the arc is kept rotating.
b) The special geometry of spiral contacts generates a radial magnetic field in all areas of the
Interrupter casing
Fixed Contact terminal Fixed Contact
Casing
Ceramic Insulator Shield
Moving Contact
Metallic Bellow Terminal
Metallic Bellow shield
Actuating Arm
Chapter 5
5.4.1.4 Vacuum Contactor
A vacuum contactor is an electromagnetic operating switch that utilizes vacuum bottle encapsulated contacts to suppress the arc. This arc suppression allows the contacts to be much smaller and uses less space than air-break contacts at higher currents.
Short circuit interruption capability is limited as back-up fuses are installed for short- circuit protection. Some contactors use a striker activated by the fuse and is provided to take care of single-phasing and short circuits, making sure to trip the electromagnetic coil.
5.4.1.4.1 Features of a Vacuum Contactor
a) Frequent operations with a vacuum interrupter b) A e de e de e e a ed
c) Wi hd a al i ible i h he c ac ON
d) Cannot be closed until the isolating contacts are fully engaged
e) Automatic shutters cover the exposed live contacts when they are withdrawn f) The contactor opens on failure of a single fuse to prevent single phasing
g) In some cases, the earthing switch automatically closes on withdrawal
Vacuum contactors are therefore very efficient at disrupting the energy of an electric arc and are used when relatively fast switching is required.
High Voltage Switchgear
Figure 5.19 Parts of the Vacuum Contactor
Figure 5.20(a) Figure 5.20(b)
Insulation Frame
Closing coil Pusher Vacuum Interrupter
Tripping Spring Aux. contacts
Moving core Pressing Spring Pressing Plate
Main Shaft
Chapter 5
The f e ed i he e l ca i a e ike fuses i.e., when the fuse blows, a pin ejects from one end such that this displacement of the striker linkage can operate a micro switch as shown in Figure 5.21.
Figure 5.21 A Striker Fuse Operating Link
Striker Fuse Contacts
Micro Switch
M
Voltage Detector
Protection Relay Surge Arrestor
ZCT for unbalanced current protection
Micro Switch contact to trip the coil in case of a single fuse failure
Striker Fuse for SC Protection
High Voltage Switchgear
5.4.2 Sulphur-hexafluoride (SF6) Circuit Breakers Sulphur-hexafluoride (SF6) gas is highly electronegative. Due to its high electronegativity, it absorbs free electrons which are produced due to arcing between the contacts of the circuit breaker.
The combination of free electrons with molecules produces heavy and big ions, which have very low mobility. Because of absorption of free electrons and the low mobility of ions. Sulphur hexafluoride has a very excellent dielectric property.
Figure 5.23
The SF6 Gas Molecule
The dielectric strength of sulphur hexafluoride gas is about 2.5 times more than that of air. SF6 is a colourless non-toxic gas, with good thermal conductivity. It does not react with materials commonly used in high voltage circuit breakers.
The combined electrical, physical, chemical and thermal properties offer many advantages when it is used in high power switchgear. Some of the outstanding properties of SF6 which make its use in high power applications desirable are:
High dielectric strength
Unique arc-quenching ability Excellent thermal stability Good thermal conductivity
The SF6 gas is identified as a low greenhouse gas. Safety regulations are being introduced in many countries to prevent its release into the atmosphere.
Figure 5.24 Greenhouse Gas Effect of SF6 gas 5.4.2.1 SF6 Gas Breaker Properties
Chapter 5
Figure 5.25 The SF6 Breaker
During the opening operation, the gas contained inside a part of the breaker is compressed by a moving cylinder that supports the contacts or by a piston. This forces the SF6 through the interrupting nozzle. When the contacts separate, an arc is established.
Arc Nozzle
Fixed Contact
Moving Contact
If the current is not very high, it is extinguished at the first zero crossing by pushing the SF6 through the arc with the help of the piston.
If the short circuit current is high, the arc extinction may not occur at the first zero crossing, but the gas pressure will increase sufficiently to blow the arc out, by connecting several interrupting heads in Figure 5.26 Arc Extinction in Gas Flow
from High to Low Pressurevia an Insulating Nozzle
High Voltage Switchgear
I he b eake c li de a d piston arrangement, the piston is fixed but the cylinder is movable. The cylinder is connected to the moving contact;
for the opening of the breaker, the cylinder along with the moving contact moves away from the fixed contact.
Due to the presence of the fixed piston, the SF6 gas inside the cylinder is compressed.
Nozzle
Fixed Contacts Moving Contacts
Fixed Piston Moving Cylinder
SF6Gas Compressed
Figure 5.27
Arrangement of Contacts in the SF6 Breaker
Figure 5.28 Operation of SF6 Circuit Breaker
Chapter 5
Due to heat convection and radiation, the compressed SF6 gas thus flows through the nozzle and over the electric arc in an axial direction.
The arc radius reduces gradually and the arc is finally extinguished at current zero. The dielectric strength of the medium between the separated contacts increases rapidly and is restored quickly as fresh SF6 gas fills the space. While quenching the arc, a small quantity of SF6 gas is broken down to some other fluorides of Sulphur like SF2 and SF4 which mostly recombine to form SF6 once again.
Upper Connection Flange Quenching Chamber Fixed extinction chamber
Fixed continuous current contact Insulating Nozzle
Moving extinction contact
Moving continuous current contact Lower connection flange
Puffer piston Contact tube
Operating lever
Figure 5.29
Operation of the SF6 Circuit Breaker
A filter is also suitably placed in the interrupter to absorb the remaining decomposed by- product. The gas pressure inside the cylinder is maintained at approximately 4 kgs per cm2. At higher pressures, the dielectric strength of the gas increases. SF6 gas pressure is always monitored by a pressure switch and in case of low pressure, the circuit breaker will trip.
5.4.2.2 Advantages
1. There is a very short arcing period due to the superior arc quenching property of SF6. 2. It can interrupt much larger currents as compared to other breakers.
3. There is no risk of fire.
4. The foundation is light and low maintenance is required 5. There are no over voltage problems.
6. There are no carbon deposits.
High Voltage Switchgear
7. Gas pressure can be monitored and in case of low pressure less than 1.5 bar, an alarm can cause the circuit breaker to trip. Charging of SF6 gas can be done in case of pressure falls in the SF6 interrupter as shown in Figure 5.30.
Figure 5.30 Gas Charging in the SF6 Circuit Breaker 5.4.2.3 Disadvantages
1. SF6 breakers are costly dueto the high cost of SF6 gas.
2. SF6 gas pressure has to be monitored as the gas is a greenhouse gas and there are other operational limitations too.
5.4.2.4 The Circuit Breaker Operating Mechanism
For the circuit-b eake e a i , b h i g-operated and stored-energy mechanisms are available. With manual spring-operated mechanisms, the closing process takes place automatically after the closing spring is charged manually.
The opening or contact springs are charged simultaneously during the closing operation,
Chapter 5
Figure 5.31 Operating Mechanism of the SF6 Circuit Breaker
Figure 5.32 Pictorial View of the MVCircuit Breaker s Operating Mechanism
To close the breaker, the closing spring can be unlatched either mechanically by means of he l cal O h b elec icall b a e e-control push button.
The closing spring charges the opening or contact pressure springs as the breaker closes.
The now discharged closing spring will be charged again automatically by the mechanism motor or manually. Then the operating sequence i.e., open-close-open is stored in the spring.
High Voltage Switchgear
For maintenance, it may be necessary to close and trip the circuit breaker locally while it is disconnected and isolated from the bus bars. The circuit-b eake / i g c l system is depicted in Figure 5.33.
Where solenoid closing is used alone, its high DC current requires the closing solenoid itself to be switched by a separate contactor.
Separate fuses are provided in each switchgear cubicle for the closing and tripping supplies so that each may be isolated without affecting the other and to provide protection to each auxiliary circuit appropriate to its loading.
M
Remote Trip Emergency
Trip Test
Switch
Trip Coil
Close Spring Close Coil
Release
Release
Trip Spring
Spring Charging
Motor Motor
Limit Switch Generator
Breaker Auxiliary Switches
Trip Relay Switches Carriage Switches
Anti-pumping Relay
Remote Close 110V +ve
110V -ve
110V +ve 6A Fuse
6A Fuse
2A Fuse
On / Off Lamps
Figure 5.33 MVCircuit Breaker s Motor / Spring Control S stem
Chapter 5
When the circuit-breaker closes, and while the control i ch i ill held i he Cl e position, an auxiliary switch completes a circuit to energise the anti-pumping relay.
The contacts of this relay change over to break the closing coil circuit and complete a holding circuit for the anti-pumping relay. Thus, the breaker will open if it is tripped by an immediate fault condition, but no further closing operations can take place until the control
i ch ha bee elea ed. Thi ide a e- h cl i g facili .
5.5 Routine Tests of Vacuum Circuit Breakers