Medium Voltage Switchgear
Switchgear includes electrical disconnect switches, fuses, or circuit breakers used to control, protect and isolate electrical equipment in an electric power system.
Switchgear system with a voltage from 3kv to 36kv is classified as medium voltage switchgear or MV switchgear. There are types of Switchgear such as metal-enclosed indoor type switchgear, metal-enclosed outdoor type switchgear, outdoor type switchgear without metal enclosure, etc. The interruption medium of this switchgear may be insulating oil, SF6 gas, or vacuum. The power network is mainly required to interrupt current during faulty conditions irrespective of what type of CB is used in the MV switchgear system.
The functions of Medium voltage switchgear are given as:
- Short circuit current interruption.
- Normal ON/OFF switching operation.
- Switching of capacitive currents.
- Switching of inductive currents.
- Some special applications.
All the above-mentioned functions must be implemented with a high degree of safety and reliability.
Short Circuit Current Interruption
Circuit breaker design mainly focuses on that all circuit breakers should be capable of interrupting short circuit current with a high degree of reliability and safety. The number of faulty tripping occurred during the total lifespan of a circuit breaker mainly relies on the location of the system, the quality of the system, and environmental conditions. In case of a much higher number of tripping, a vacuum circuit breaker is the best choice as it may not require any maintenance up to 100 faulty tripping with a short circuit current up to 25 KA. Whereas, other circuit breakers need to be maintained after 15 to 20 faulty tripping with the same short circuit current of CB. The substations reunited in rural areas are generally of an outdoor type, and most of them are the unattended type. Hence for this type of application maintenance-free outdoor type, medium voltage switchgear is most suitable. Porcelain clad vacuum circuit breaker meets this demand against the conventional indoor kiosks.
Switching of Capacitive Current
The function of the capacitor bank in a medium voltage power system is to improve the power factor of the system. Unloaded overhead lines and Unloaded cables have also capacitive charging current and should be disconnected from the system safely without re-ionization. overvoltage is caused by Re-ionization in the contact gap in the system. The vacuum circuit breaker meets the requirement.
While switching on a capacitor bank, a high rate of rising of making current will flow through the CB contacts. A circuit breaker with liquid quenching medium and tulip contacts may suffer from contact pin retardation. Vacuum medium voltage switchgear is the most perfect choice for this purpose, as vacuum circuit breaker has low electric arcing during short pre arcing time.
Switching of Inductive Current
Older VCB had a current chopping level of 20 A when these breakers were used to switch transformers, special surge protection devices were required. Modern VCB has a very low chopping current which is about 2 – 4A. Hence modern vacuum medium voltage switchgear is very much suitable for switching an unloaded transformer. As the modern VCB chops the current at a very low level, there is no need for additional surge protection devices. Hence VCB is suitable for very low inductive load switching. But when the inductive current in the system is low but not very low VCB is the best choice.
Special Application of Medium Voltage Switchgear
Railway Traction
Medium voltage switchgear also is applied for a single-phase railway track system. The main function of the circuit breaker associated with the railway traction system is to interrupt the short circuits, on the overhead catenary system which occurs frequently and it is transient. Hence, a circuit breaker used for this purpose should have, short braking time for small contact gap, short arcing time, quick breaking, and VCB is the best possible solution. Arcing energy is much higher in the single-phase CB than 3 phase CB. It is still much lower in a vacuum circuit breaker than that in a conventional circuit breaker. The number of short circuits that occur in the overhead catenary system is much higher than those occurring in the electrical transmission system. Medium voltage switchgear with a vacuum circuit breaker is most suitable for traction application. We can conclude that, in the medium-voltage system where the tripping rate is very high, MV Vacuum Switchgear is the most suitable solution.
Arc Furnace
An electric arc furnace is required to be switched OFF and ON frequently. The current to be switched may be from 0 to 8 times the rated current of the furnace. An electric arc furnace is to be switched ON and OFF at its normal rated current up to 2000A, around 100 times per day. A normal, SF6 circuit breaker, air circuit breaker, and the oil circuit breaker is not at all economical for this frequent operation. A standard vacuum circuit breaker is the most suitable alternative for this frequent high current circuit breaker operation.
