From 3 KV to 36 KV switchgear system is categorized as medium voltage switchgear or MV switchgear. These switchgears are of many types. They may metal enclosed indoor type, metal enclosed outdoor type, outdoor type without metal enclosure, etc. The interruption medium of this switchgear may be oil, SF6 and vacuum. The main requirement of MV power network is to interruptcurrent during faulty condition irrespective of what type of CB is used in the MV switchgear system. Although it may be capable of functioning in other conditions also.Medium voltage switchgear, should be capable of,
1. Normal ON/OFF switching operation.
2. Short circuit current interruption.
3. Switching of capacitive currents.
4. Switching of inductive currents.
5. Some special application.
All the above mentioned function must be carried out with high degree of safety and reliability.
The main focus of circuit breaker design is to that all circuit breaker should be capable of interrupting short circuit current with high degree of reliability and safety. The number of faulty tripping occurred during total life span of a circuit breaker mainly depends upon location of the system, quality of system and environment condition. If the number of tripping is much high, the best choice is vacuum circuit breaker as it may not require any maintenance up to 100 faulty tripping with short circuit current up to 25 KA. Whereas other circuit breakers require maintenance after 15 to 20 faulty tripping with same short circuit current of CB.
The substations reunited in rural areas are generally of outdoor type, and most of them are unattended type. Hence for this type of applications maintenance free outdoor type, medium voltage switchgear is most suitable. Porcelain clad vacuum circuit breaker meets this demand against the conventional indoor kiosks.
The capacitor bank is used in medium voltage power system to improve power factor of the system. Unloaded cable and unloaded overhead lines has also capacitive charging current. The capacitor bank and unloaded power lines should be disconnected from the system safely without re-ionization. Re-ionization in the contact gap causes over voltage in the system. Vacuum circuit breaker meets the requirement. While switching on a capacitor bank, a high rate of rise of making current will flow through the CB contacts. Circuit breaker with liquid quenching medium and tulip contacts may suffer from contact pin retardation. Vacuum medium voltage switchgear is most perfect choice for this purpose, as vacuum circuit breaker has low arcing during short pre arcing time.
Older VCB had current chopping level of 20 A when these breakers were used to switch transformers, special surge protection device were required. Modern VCB has very low chopping current which is about 2 - 4A. Hence modern vacuum medium voltage switchgear is very much suitable for switching unloaded transformer. As the modern VCB chops the current at very low level, there is no risk of 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
An arc furnace is required to be switch off and on frequently. The current to be switched may be from 0 to 8 times of the rated current of the furnace. An 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 oil circuit breaker is not at all economical for this frequent operation. Standard vacuum circuit breaker is most suitable alternative for this frequent high current circuit breaker operation.
Another application of medium voltage switchgear is single phase railway track system. The main function of the circuit breaker associated with railway traction system, is to interrupt short circuit, on the overhead catenary system which occur frequently and are transient in nature. Hence circuit breaker used for this purpose should have, short breaking time for small contact gap, short arcing time, quick breaking, and VCB is the best possible solution. Actually 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 conventional circuit breaker. The number of short circuit occurs in the overhead catenary system is much higher than those occurring on electrical transmission system. Medium voltage switchgear with vacuum circuit breaker is most suitable for traction application.
It can be concluded that, in medium voltage system where tripping rate is very high, MV Vacuum Switchgear is most suitable solution.