Low Voltage Switchgear or LV Switchgear
Generally electrical switchgear rated upto 1 KV is termed as low voltage switchgear. The term LV Switchgear includes low voltage circuit breakers, switches, off load electrical isolators, HRC fuses, earth leakage circuit breaker, miniature circuit breakers (MCB) and molded case circuit breakers (MCCB) etc i.e. all the accessories required to protect the LV system.
The most common use of LV switchgear is in LV distribution board. This system has the following parts
The incomer feeds incoming electrical power to the incomer bus. The switchgear used in the incomer should have a main switching device.
The switchgear devices attached with incomer should be capable of withstanding abnormal current for a short specific duration in order to allow downstream devices to operate. But it also be cable of interrupting maximum value of the fault current generated in the system. It must have interlocking arrangement with downstream devices. Generally air circuit breakers are preferably used as interrupting device. Low voltage air circuit breaker is preferable for this purpose because of the following features
3.High normal current rating up to 600 A
4.High fault withstanding capacity upto 63 kA
Although air circuit breakers have long tripping time, big size, high cost but still they are most suitable for low voltage switchgear for the above mentioned features.
Sub – Incomer
Next downstream part of the LV Distribution board is sub – incomer. These sub – incomers draw power from main incomer bus and feed this power to feeder bus. The devices installed as parts of a sub – incomer should have the following features
1.Ability to achieve economy without sacrificing protection and safety
2.Need for relatively less number of inter – locking since it cover limited are of network. ACBs and switch fuse units are generally used as sub – incomers along with molted case circuit breakers(MCCB)
Different feeders are connected to the feeder bus to feeds different loads like, motor loads, lighting loads, industrial machinery loads, air conditioner loads, transformer cooling system loads etc. All feeders are primarily protected by switch fuse unit and in addition to that, depending upon the types of load connected to the feeders, the different switchgear devices are chosen for different feeders. Let’s discuss in details
2.Motor feeder should be protected against over load, short circuit, over current up to locked rotor condition and single phasing.
1.Industrial Machinery Load Feeder
2.Feeder connected industrial machinery load like oven, electroplating bath etc are commonly protected by MCCBl andfswitch fuse units
3.Lighting Load Feeder
This is protected similar to industrial machinery load but additional earth leakage current protection is provided in this case to reduce any damage to life and property that could be caused by harmful leakages of current and fire.
In LV switchgear system, electrical appliances are protected against short circuit and over load conditions by electrical fuses or electrical circuit breaker. However, the human operator is not adequately protected against the faults occurs inside the appliances. The problem can be overcome by using earth leakage circuit breaker. This operates on low leakage current. The earth leakage circuit breaker can detect leakage current as low as 100 mA and is capable of disconnecting the appliance in less than 100 msec.
Medium Voltage Switchgear
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, SF and vacuum. The main requirement of MV power network is to interrupt current 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.A 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.
Short Circuit Current Interruption
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 trippings with short circuit current up to 25 KA. Whereas other circuit breakers require maintenance after 15 to 20 faulty trippings 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.
Switching of Capacitive Current
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.
Switching of Inductive Current
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 to be switched on and off at its normal rated current up to 2000A, around 100 times per day. A normal, SF 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.