Switchgear Protection
Definition of Switchgear
A switchgear is a system includes all the switching devices associated with control, metering and regulating of electrical power systems ,thus providing power system protection. The Assembly of such devices in a logical manner forms switchgear. In other words systems used for switching, controlling and protecting the electrical power circuits and different types of electrical equipment are known as switchgear. This is very basic definition of switchgear.
Switchgear and Protection
In same way every electrical circuit including high voltage electrical power system needs switching and protective devices. But in high voltage and extra high voltage system, this switching and protective scheme becomes complicated one for high fault current interruption in safe and secure way. In addition to that from commercial point of view every electrical power system needs measuring, controling and regulating arrangement. Collectively the whole system is called switchgear and protection of power system. The electrical switchgear has been developing in various forms.
Switchgear protection plays a vital role in modern power system network, right from generation through transmission to distribution end. The current interruption devices are called circuit breaker. The circuit breakers can be operated manually as when required and it can also be operated autometically during over current and short circuit or any other faults in the system by sensing the abnormality of system parameters. These power system parametrs can be current, voltage, frequency, phase angle etc. The circuit breaker senses the faulty condition of system through protection relays and these relays are again actuated by faulty signal normally comes from current transformer or voltage transformer.
The switchgear has to perform the function of carrying, making and breaking the normal load current like a switch and it has to perform the function of clearing the fault in the power system. 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, current transformers, voltage transformers, protection relays, measuring instruments, electrical switches, electrical fuses, miniature circuit breaker, lightning arresters or surge arresters, electrical isolators and other associated pieces of equipment.
Electric switchgear is necessary at every switching point in the electrical power system. There are various voltage levels and hence various fault levels between the generating stations and load centers. Therefore various types of switchgear assembly are required depending upon different voltage levels of the system. Besides the power system networks, electrical switchgears are also required in industrial works, industrial projects, domestic and commercial buildings.
High Voltage Switchgear
The power system deals with voltage above 36KV, is referred as high voltage switchgear. As the voltage level is high the arcing produced during switching operation is also very high. 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, hence high voltage circuit breaker (CB) should have special features for safe and reliable operation. Faulty tripping and switching operation of high voltage circuit are very rare. Most of the times, 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 (MOCB), air blast circuit breaker and SF6 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 SF6 circuit breaker, single pressure SF6 circuit breaker and two pressures SF6 circuit breaker. The single pressure system is the state of art for high voltage switchgear system, in present time. Nowadays SF6 gas as arc quenching medium, has become most popular for high and extra high voltage electrical power system. Although, SF6 gas contributes to the greenhouse effect. It has 23 times stronger impact on the greenhouse effect, than that of CO2. Hence, leakage of SF6 gas during the service life of 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 circuit breaker in 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.
On the other hand, SF6 circuit breaker has the major advantage of low maintenance. High voltage switchgears are categorized as,
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 (assembly of interrupters) is located, with suitable insulator supports inside a metallic vessel(s) of ground potential, filled with insulating medium. In 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.
There are mainly three types of circuit breaker, as we said earlier, used in high voltage switchgear system i.e. air blast circuit breaker, SF6 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 insulating oil requirement can be minimized by placing the interrupting units in an insulating chamber at the live potential on an insulator column.
SF6 Circuit Breaker
SF6 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 SF6, 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 switchgear in high voltage system.
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 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.
Essential Features of High Voltage Circuit Breaker
The essential features to be provided 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.
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 the extinction of arc transient recovery voltage appears across the CB contacts, with high frequency. This transient recovery voltage ultimately approaches to open circuit voltage.
This recovery voltage can be represented as 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 a combination of many different frequencies due to the 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 the 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 rising of transient recovery voltage can be limited. The dielectric recovery of the arc quenching medium and rate of rising of transient recovery voltage has great influence on the performance of the circuit breaker used in high voltage switchgear system. In an air blast circuit breaker, once ionized air is de-ionized very slowly, thus air takes a long time to recover dielectric strength. That is why it is preferable very slowly, thus air takes a long time to recover dielectric strength. That is why it is preferable to used low-value breaker resistor to slow down the rate of rising of recovery voltage. On the other hand ABCB is less sensitive to the initial recovery voltage because of high arc voltage in SF6 circuit breaker, the interrupting medium (SF6) has faster rate of recovery of dielectric strength, than air. Lower arc voltage makes SF6 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 5 km 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.
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 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 main requirement of 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.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 functions must be carried out with a high degree of safety and reliability.
Short Circuit Current Interruption
The main focus of circuit breaker design is to 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 depends upon the location of the system, the quality of the system and environment conditions. 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 an outdoor type, and most of them are the 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 electric 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 an unloaded transformer. As the modern VCB chops the current at a very low level, there is no need 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 electric arc furnace is required to be switched OFF and ON frequently. The current to be switched may be from 0 to 8 times of 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 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 occurs frequently and it is transient. Hence, a 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. 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 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. We can conclude that, in the medium voltage system where the tripping rate is very high, MV Vacuum Switchgear is the most suitable solution.
Low Voltage Switchgear or LV Switchgear
Published on 24/2/2013 & updated on 25/9/2018
Generally electrical switchgear rated upto 1KV 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 should be cable of interrupting maximum value of the fault current generated in the system. It must have an 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.
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 interlocking since it cover limited are of network.
ACBs (Air Circuit Breakers) 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 us discuss in details
· Motor Feeder
· Motor feeder should be protected against over load, short circuit, over current up to locked rotor condition and single phasing.
· Industrial Machinery Load Feeder
· Feeder connected industrial machinery load like oven, electroplating bath etc are commonly protected by MCCBl and switch fuse disconnector units
· 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 overload 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 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 incomer comes from 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. 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. These switches are so interlocked along with bus section switch or bus coupler that only one incomer switch can be put on if bus section switch is in on position and both sub incomer switches can be put on only if bus section switch is at off position. This arrangement is fruitful for preventing any mismatch of phase sequence between the sub - incomers. The different load feeders are connected to any of both sections of the feeder bus. Here motor feeder is protected by thermal overload device along with conventional switch fuse unit. Heater feeder is protected only by conventional switch fuse unit. The domestic lighting and AC loads are separately protected by a miniature circuit breaker along with common conventional switch fuse unit. This is most basic and simple scheme for low voltage switchgear or LV distribution boar
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