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
Incomer
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
1.Simplicity
2.Efficient performance
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)
Feeders
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
1.Motor Feeder
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
Arc Furnace
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.
Railway Traction
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.
High Voltage Switchgear
The power system deals with voltage above 36KV, is referred as high voltage. As the voltage level is high the arcing produced during switching operation is also very high. So, special care to be taken during designing of high voltage switchgear. High voltage circuit breaker, is the main component of HV switchgear, hence high voltage circuit breaker should have special features for safe and reliable operation. Faulty tripping and switching operation of high voltage circuit are very rear. Most of the time these circuit breakers remain, at ON condition, and may be operated after a long period of time. So CBs must be reliable enough to ensure safe operation, as when required. High voltage circuit breaker technology has changed radically in the last 15 years. Minimum oil circuit breaker, air blast circuit breaker and SF circuit breaker are mostly used for high voltage switchgear.
Vacuum circuit breaker is rarely used for this purpose as till date vacuum technology is not adequate for interrupting very high voltage short circuit current. There are two types of SF circuit breaker, single pressure SF breaker and two pressures circuit breaker. The single pressure system is the state of art for high voltage switchgear system, in present time. Now days SF gas as arc quenching medium, has become most popular for high and extra high voltage electrical power system. Although, SF gas has strong impact on the greenhouse effect. It has 23 times stronger impact on the greenhouse effect, than that of CO. Hence, leakage of SF gas during the service life of circuit breaker must be prevented. In order to minimize the emission of SF gas, the N – SF and CF – SF gas mixture, may be used in circuit breaker in future, as substitute of pure SF. It must always be taken care of that, no SF gas comes out in atmosphere during maintenance of the CB.On the other hand, SF circuit breaker has the major advantage of low maintenance.
High voltage switchgears are categorized as,
1.Gas insulated indoor type,
2.Air insulated outdoor type.
Again, outdoor type air insulated circuit breakers are classified as,
1.Dead tank type circuit breaker
2.Live tank type circuit breaker
In dead tank type CB, the switching device is located, with suitable insulator supports inside a metallic vessel(s) at ground potential filled with insulating medium.
In live tank circuit breaker, the interrupts are located in an insulated busing, at the system potential. The live tank circuit breakers are cheaper and required less mounting space.
There are mainly three types of circuit breaker, as we said earlier, used in high voltage switchgear system i.e. air blast circuit breaker, SF circuit breaker, oil circuit breaker. And vacuum circuit breaker is rarely used.
Air Blast Circuit Breaker
In this design, a blast of high pressure compressed air is used to quench arc between two detaching contacts, when the arc column ionization is least at currents zero.
Oil Circuit Breaker
This is further classified as bulk oil circuit breaker (BOCB) and minimum oil circuit breaker (MOCB). In BOCB, the interrupting unit is placed inside an oil tank of earth potential. Here oil is used as both insulating and interrupting medium. In MOCB on the other hand, the oil requirement can be minimized by placing the interrupting units in an insulating chamber at live potential on an insulator column.
SF Circuit Breaker
SF gas is widely used as arc quenching medium in HV applications today. Sulfur hexafluoride gas is a high electronegative gas having excellent dielectric and arc quenching properties. High dielectric and insulating properties of SF, make it possible to design high voltage circuit breaker with smaller overall dimension, shorter contact gap. Excellent insulating property helps to design and construct indoor type high voltage switchgear.
Vacuum Circuit Breaker
In 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 next zero crossing 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, but till it is not a suitable solution for high voltage switchgear, as VCB made for very high voltage level is not economical at all.
The essential features to be provide 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,
1.Terminal Faults
2. Short Line Faults
3.Transformer or reactors magnetizing current.
4.Energizing long transmission line.
5.Charging capacitor bank.
6.Switching of out of phase sequence.
Terminal Fault
Generally the load connected to the power system is inductive in nature. Due to this inductance, when short circuit current is just interrupted by a circuit breaker, there is a chance of high restriking voltage of high frequency oscillation in order of few hundred Hz.
This voltage has two parts
1.Transient recovery voltage with high frequency oscillation immediately after the arc extinction.
2.After die down this high frequency oscillation, power frequency recover voltage appears across the CB contacts.
Transient Recovery Voltage
Just after extinction of arc transient recovery voltage appears across the CB contacts, with high frequency. This transient recovery voltage ultimately approaches to open circuit voltage.
The frequency of oscillation is governed by the circuit parameter L and C.
The resistance present in the power circuit damps out this transient voltage. The transient recovery voltage has not a single frequency, it is combination of may different frequencies due to complexity of the power network.
Power Frequency Recovery Voltage
This is nothing but open circuit voltage appears across the CB contacts, just after the transient recovery voltage damped out. In three phase system the power frequency recovery voltage differs in different phase. It is highest in first phase. If the network neutral is not earthed, the voltage across the first pole to be cleared is 1.5U where U is the phase voltage. In an earthed neutral system it will be 1.3U.
By using damping resistor, the magnitude and rate of rise of transient recovery voltage can be limited. The dielectric recovery of the arc quenching medium and rate of rise of transient recovery voltage has great influence on the performance of the circuit breaker used in high voltage switchgear system.
In air blast circuit breaker, once ionized air is deionized very slowly, thus air takes long time to recover dielectric strength. That is why it is preferable to used low value breaker resistor to slow down rate of rise of recovery voltage. On the other hand ABCB is less sensitive to the initial recovery voltage because of high arc voltage in SF circuit breaker, the interrupting medium (SF) has faster rate of recovery of dielectric strength, than air. Lower arc voltage, makes SF CB more sensitive to the initial recovery voltage.
In oil circuit breaker, during arc having pressurized hydrogen gas (produced during recombination of oil due to arc temperature) provides quick recovery of dielectric strength immediate after current zero. Hence OCB is more sensitive to rate of rise of recovery voltage. It is also more sensitive to initial transient recovery voltage.
Short Line Fault
Short line fault in transmission network is defined as the short circuit faults occurred, within 5km of the line length. Double frequency being impressed on the circuit breaker and the difference of source and line side transient recovery voltage. Both voltages start from instantaneous values at the opposition of the circuit breakers prior to the interruption. On the supply side the voltage will oscillate at supply frequency and ultimately approaches to open circuit voltage. On the line side, after interruption, trapped charges initial traveling waves through the transmission line, since there is no driving voltage on the driving side, the voltage ultimately becomes zero because of the line losses.