Switch Gear-1 Low Voltage Switchgear or LV Switchgear:
Generally, electrical switchgear rated up to 1KV is termed as low voltage switchgear in the bay. The term LV Switchgear contains the low voltage circuit breakers, and the switches, and the offload electrical isolators, and HRC fuses, and the earth leakage circuit breakers, miniature circuit breakers (MCB), and molded case circuit breakers (MCCB), etc i.e. all the accessories required to protect the LV system in the system. The most common use of LV switchgear is in the LV distribution board in the system. This system has the following parts of the switchgear
The incomer feeds incoming electrical power to the incomer bus in the method. The switchgear used in the incomer should have the main switching device in the 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 to perform the operation. But it should be cable of interrupting maximum value of the fault current generated in the system which detects the system. It must have an interlocking arrangement with downstream devices of the method. Generally, air circuit breakers are preferably used as an interrupting device in the following method. A low voltage air circuit breaker is preferable for this purpose because of the following features the system.
High normal current rating up to 600 A
High fault withstanding capacity up to 63 kA
Although air circuit breakers have long tripping time, and big size, and high cost but still they are most suitable for low voltage switchgear for the above-mentioned features of the system.
Sub – Incomer
The next downstream part of the LV Distribution board is sub – income of the system. These sub-incomers draw power from the main incomer bus and feed this power to the feeder bus of the system. The devices installed as parts of a sub – incomer should have the following features of the switchgear
Ability to achieve economy without sacrificing protection and safety and Need for relatively less number of interlocking since it covers limited are off the network of the system.
ACBs (Air Circuit Breakers) and switch fuse units are generally used as a sub – incomers along with molted case circuit breakers (MCCB) in the system.
Different feeders will be connected to the feeder bus to feeds different loads like motor loads, and lighting loads, and industrial machinery loads, and air conditioner loads, and transformer cooling system loads, etc. All feeders are primarily protected by switch fuse units and in addition to that, and the types of load connected to the feeders, the different switchgear devices are chosen for different feeders of the switchgear. Let us discuss in details
The motor feeder should be protected against overload, short circuit, over-current up to locked rotor condition, and single phasing.
Industrial Machinery Load Feeder
Feeder connected industrial machinery load like an oven, electroplating bath, etc are commonly protected by MCCBl and switch fuse disconnector units in the system.
Lighting Load Feeder
This is protected similar to industrial machinery load but additional earth leakage which will use the 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 of the system.
In the LV switchgear system, electrical appliances are protected against short circuit and overload conditions by electrical fuses or electrical circuit breakers. However, the human operator is not adequately protected against the faults occurs inside the appliances. The problem can be overcome by using an 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.
A typical diagram of low voltage switchgear is shown above. Here the main income comes from the LV side of an electrical transformer. This incomer through an electrical isolator as well as an MCCB (not shown in the figure) feeds the incomer bus of the system. Two sub-incomers are connected to the incomer bus and these sub-incomers are protected by means of either switch fuse unit or air circuit breaker in the switchgear. These switches are so interlocked along with bus section and the switch and the bus coupler that only one incomer switch can be put on if bus section switch is in on position then the income will get the off position of the system and the on condition. This arrangement is fruitful for preventing any mismatch of phase sequence between the sub – incomers in the system. The different load feeders are connected to any of both sections of the feeder bus in this range. Here motor feeder is protected by a thermal overload device along with the conventional switch fuse unit in the system. Heater feeder is protected only by the conventional switch fuse unit of the system. The domestic lighting and AC loads are separately protected by a miniature circuit breaker along with a common conventional switch fuse unit in the system. This is the most basic and simple scheme for low voltage switchgear or LV distribution board of the system.
Definition of Switchgear
A switchgear is a generic term that includes all the switching devices associated with power system protection in the range. It also includes all devices associated with control, metering, and regulating electrical power systems for current. Assembly of such devices in a logical manner forms switchgear will remain the same in the switchgear. In other words, systems used for switching, controlling, and protecting contain the power circuit, and different types of electrical equipment are known as switchgear of the system . This is a very basic definition of switchgear contains the same.
The switchgear has to perform the function of carrying, making, and breaking the full load current and load current like a switch and it has to perform the function of clearing the fault in the power system of the device. In addition to that, it also has the provision of metering and regulating the various parameters of electrical power systems. Thus the switchgear includes circuit breakers, and current transformers, and voltage transformers, and protection relays, and measuring instruments, electrical switches, and electrical fuses, and miniature circuit breakers, and lightning arresters and surge arresters, electrical isolators and other associated pieces of equipment in the system.
Electric switchgear is necessary at every switching point in the electrical power system in the bay. There are various voltage levels and hence various fault levels between the generating stations and load centers of the range. Therefore various types of switchgear assembly are required depending on the different voltage levels of the system in the range. Besides the power system networks, electrical switchgear is also required with industrial projects, domestic and commercial buildings of the region.
The power system deals with voltage above 36KV, is referred to as high voltage switchgear. As the voltage level is high the arcing produced during switching operation is also very high I the system. So, special care is to be taken during the designing of high voltage switchgear. High voltage circuit breaker, is the main component of HV switchgear, And the high voltage circuit breaker will have special features for safe and reliable operation in the system. Faulty tripping and switching operation of high voltage circuits are very rare in the region. Most of the time, these circuit breakers remain an ON condition and maybe operated after a long period of time contains the system. So CBS must be reliable enough to ensure safe operation, as when required it will get high. High voltage circuit breaker technology has changed radically in the last 15 years of the breaker. Minimum oil circuit breaker (MOCB), air blast circuit breaker, and SF6 circuit breaker are mostly used for high voltage switchgear in the system.
A vacuum circuit breaker is rarely used for this purpose as to date vacuum technology is not adequate for interrupting very high voltage short circuit current of the system. There are two types of an SF6 circuit breaker, single pressure SF6 circuit breaker and two pressures SF6 circuit breaker. The single pressure system is the state of art for a high voltage switchgear system, in the present time. Nowadays SF6 gas as arc quenching medium has become most popular for high and extra-high voltage electrical power systems. Although, SF6 gas contributes to the greenhouse effect. It has 23 times stronger impact on the greenhouse effect than that of CO2 in the breaker. Hence, leakage of SF6 gas during the service life of the circuit breaker must be prevented. In order to minimize the emission of SF6 gas, the N2 – SF6 and CF4 – SF6 gas mixture may be used in the circuit breaker in the future, as the substitute of pure SF6. It must always be taken care of that, no SF6 gas comes out in atmosphere during maintenance of the CB in the system.
Dead Tank Type Circuit Breaker
Live Tank Type Circuit Breaker
In dead tank type CB, the switching device (assembly of interrupters) is located, with suitable insulator supports inside a metallic vessel(s) of ground potential, filled with an insulating medium. In the live tank circuit breaker, the switching device (assembly of interrupters) is located on insulated bushings, at the system potential. The live tank circuit breakers are cheaper and required less mounting space of the system.
There are mainly three types of circuit breakers, as we said earlier, used in high voltage switchgear system i.e. air blast circuit breaker, SF6 circuit breaker, oil circuit breaker, and the vacuum circuit breaker is rarely used.
In this design, a blast of high pressure compressed air need to use the arc between two detaching contacts, when the arc column ionization is least at currents zero of the system.
Oil Circuit Breaker
This is further classified as bulk oil circuit breaker (BOCB) and minimum oil circuit breaker (MOCB) in the fuse. In BOCB, the interrupting unit is placed inside an oil tank of earth’s potential system in the fuse. Here oil is used as both insulating and interrupting medium in the MCB. In MOCB on the other hand, the insulating oil requirement can be minimized by placing the interrupting units in an insulating chamber at the live potential on an insulator column of the system.
SF6 Circuit Breaker
SF6 gas is widely used as an arc quenching medium in HV applications today in the range. Sulfur hexafluoride gas is a high electronegative gas having excellent dielectric and arc quenching properties of the system. High dielectric and insulating properties of SF6, and it is used to design high voltage circuit breakers with smaller overall dimensions, a shorter contact gap of the system. Excellent insulating property helps to design and construct indoor type switchgear in a high voltage system of the range.
Vacuum Circuit Breaker
In a vacuum, there is no further ionization between two separated current-carrying contacts, after current zero. The initial arc is caused by it will die as soon as the next zero-crossings but as there is no provision of further ionization once the current is crossed its first zero, the arc quenching is completed. Although the arc quenching method is very fast in VCB, it is not a suitable solution for high voltage switchgear, as VCB made for a very high voltage level is not economical at all.
The essential features to be provide
d in high voltage circuit breaker, to ensure safe and reliable operation the breakers used in high voltage switchgear, must be capable of being operated safely for,
Transformer or reactors magnetizing current.
Energizing long transmission lines.
Charging capacitor bank.
So, special care is to be taken during the designing of high voltage switchgear. High voltage circuit breaker, is the main component of HV switchgear, And the high voltage circuit breaker will have special features for safe and reliable operation in the system. Faulty tripping and switching operation of high voltage circuits are very rare in the region. Most of the time, these circuit breakers remain an ON condition and maybe operated after a long period of time contains the system.