Vacuum Circuit Breakers
for Electric Arc Furnace heavy duty switching applications
Vacuum Circuit breakers use Vacuum as the arc quenching media. Vacuum also offers the highest insulating strength & hence far superior in arc quenching properties than any other medium. This technology is highly suitable for medium voltage applications...
- C V Govinda Raju
Electrical Circuit Breaker is a switching device, operated manually as well as automatically, used for controlling & protection of Electrical Power system & industrial processes. As modern systems calls for handling huge currents, special design considerations are needed for safe interruption of the Arc during its switching operations.
Types of Circuit Breakers
Classification based on Voltage levels.
1. Low Voltage Circuit Breakers (< 1 kV)
2. Medium Voltage Circuit Breakers (1-72 kV)
3. High Voltage Circuit Breakers (> 72 kV)
4. Vacuum Circuit Breaker
Air Blast Circuit breakers use high pressure air blast as an arc quenching medium. Under normal conditions contacts remain closed. When a fault occur, contacts are opened & an arc is struck between them. Opening of contacts are done by a flow of air blast. Further the air blast cools the arc & sweeps away the arching products into the atmosphere. Thus the dielectric strength of the medium between the contacts increases, preventing the arc from re-establishing. The arc gets extinguished & the flow of current is interrupted.
SF6 Circuit Breakers use Sulphur hexa flouride gas as an arc quenching medium. SF6 gas is an Electro negative gas which has a strong tendency to capture & absorb the conducting free Electrons from the arc when the contacts separate. Loss of conducting electrons from the arc builds enough insulation to extinguish the arc. These breakers are very effective for high power & high voltage applications.
Vacuum Circuit breakers use Vacuum as the arc quenching media. Vacuum also offers the highest insulating strength & hence far superior in arc quenching properties than any other medium. This technology is highly suitable for medium voltage applications.(See Fig.1) When the contacts of the VCB (Fig.2) are opened in vacuum, an arc is produced between the contacts by the ionization of metal vapors of Contacts. However the arc is extinguished quickly because the metallic vapor, electrons & ions produced during arc, condense quickly on the surface of the contacts resulting in faster recovery of Dielectric strength.
Electric Arc Furnace
The duty cycle demanded by the working of Electric Arc Furnaces (EAF) in Steel industries, pose very special demands on the Circuit breakers that perform the switching operations of the furnace Transformers.
A.C. Electric Arc Furnaces with their Non-linear characteristic, highly erratic & asymmetrical load constitute “Dirty loads” on utilities, resulting in voltage distortions (deviating from their basic Sinusoidal voltage form), dynamic & Static Voltage variations, very poor & fluctuating power factor, harmonic distortions etc.. pose challenges to very weak Electricity Grids. In view of the large power requirements of the EAF’s, they are connected to Medium voltage networks. Circuit breakers used in these utility power distribution networks feeding power to these Steel plants experience infrequent switching. Operational reliability after years of inactivity, against a fault, is a must for breakers used in the distribution networks. With the extensive usage of Ultra High Power Electric Arc Furnaces for Steel making, use Furnace transformers of rating around 100 MVA & above typically connected to 30 kV level plant buses with fault levels of up to 1500 MVA, resulting in Breaker currents of several thousand Amperes & fault currents up to 40kA.
But the circuit breakers used for controlling the day to day operations of the Electric Arc Furnace at the Furnace end, need to execute about 100 ‘Full Load’ operations every day without causing dangerous switching ‘over voltages’, without causing excessive wear on the contacts & operating mechanisms. This kind of stringent working requirements can be met out only by Vacuum Circuit Breakers & no wonder, Steel Plants all over the world have switched over to VCB’s for their Electric Arc Furnace controls. As a word of caution, a ‘custom designed’ surge suppression circuit with properly designed R-C network need to be installed with each VCB controlling the Furnace Transformer. These heavy duty VCB’s are rated for 120,000 mechanical operations & the maintenance free vacuum bottles are designed for 30,000 full load interruptions. That is annually, one vacuum bottle replacement & three lubrication services would be required on the Arc furnace VCB’s, practically making it maintenance free. This ensures maximum availability of the breakers with least interruptions when compared to other types of Circuit breakers.
Since the arc furnace represent dynamic loads they produce higher order frequencies (Harmonics) & load currents can be highly asymmetrical in the three phases, which need to be successfully handled by the furnace switchgear with absolute reliability & without harmful side effects on the power system.
Furnace Switching – “ON” operation using the VCB
All the three poles of the VCB will not generally close at the same instant. The first closing contact produces high - frequency transients because in that instant the line side Capacitance interact with the load side capacitance & bus bar inductivity forming resonant circuits. The frequency & amplitude of these transients largely depend on the individual installation parameters such as bus bar inductances, Cable capacitances, Stray capacitance of the equipment etc. Such high frequency transients can, however, excite resonances with certain natural frequencies of the Furnace transformer windings & then the resulting voltage amplification can be dangerous for the insulation of the Furnace transformer winding. Such resonances are more likely with increased cable lengths (Capacitances) on the load side of the breaker. In order to avoid / reduce the ill effects of these voltage transients, use of properly designed R-C network & surge suppressors for a particular Furnace installation would be required. The designers would need the following basic technical parameters for a particular installation.(See Fig.3)
a. Max. Fault level (MVA) of the Medium voltage bus bar to which the Furnace feeder is connected.
b. Number of parallel Runs, length of each run & size of the MV cables connecting the VCB at the MV switch yard & the Furnace controlling VCB at the Furnace end.
c. Size, length & no. of runs of cables if used or the size of Copper bus bars, no. of bus bars in parallel (if bus bars are used), between the Furnace controlling VCB to the Furnace Transformer Primary bushings.
During the planning stage of the Steel Plant, minute details such as the above, will not be generally available. Hence ordering of the R-C network & Surge suppressor can be done when once the installation is in progress and details are available to have a properly designed system. Alternatively the best recommendation is to install the Furnace Controlling breaker very close to the furnace transformer as possible & to use air insulated Copper bus bars rather than insulated cables for the interconnection of furnace controlling VCB & Furnace transformer.
Furnace Switching – “OFF” operation using the VCB
When we are switching “OFF” the operating Furnace, we are virtually interrupting inductive circuits. Here the type of the Circuit breaker & its arc quenching system plays an important factor. Here comes the benefit of VCB’s which have the ability to interrupt small as well as very large currents even of high frequencies with very high di/dt values, extremely fast, which otherwise lead to surges & transients which need to be controlled within the permissible limits of the electrical system.
The phenomena leading to voltage surges during switching “OFF” of the furnace are:
• Current Chopping: This occur when the currents are interrupted prior to reaching a natural Zero. Surge voltage resulting from chopping can be handled safely by the usage of properly designed Surge Arrestors mounted very close to the Furnace Transformer bushings. In the case of Furnace control breakers, other than VCB’s, if the time taken to build the di-electric strength of the gap is long, then re-ignition across the open gap is inevitable. In the case of VCB’s this time constant being very short, even high frequency currents can thus be interrupted.
• Ignition & Multiple re-ignitions.
Multiple re-ignition produced due to high frequency transients during opening of the breaker ,can be reduced by properly designing the R-C network, & adopting low impedance grounding, taking into consideration the Electrical system configuration of the particular plant (See fig.3).
Not all the three contacts of any circuit breaker open exactly at the same instant & with high number of operational cycles of Electric Arc Furnace Breakers, the harmful effects of the above phenomena of voltage surges need serious consideration. Here comes the specially adapted & field proven Vacuum Circuit Breakers which ensures reliable Arc Furnace switching duty & complete power system protection, thereby increasing the reliability of the Steel Plant. Today, more than 70% of all circuit-breakers installed in medium-voltage systems are based on vacuum switching technology.
C V Govinda Raju
(Former) Executive Director
Karnataka Vidyuth Karkhane Ltd.
ISPAT Group of Companies.
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Vacuum Circuit Breakers
for Electric Arc Furnace heavy duty switching applications.