Introduction to Switchgear

Hello Electrical Engineers. Today in my very first article I am going to introduce you to the subject of the Switchgear as well as protection, because both are closely related to each other.

First of all the question arises is that what actually is Switchgear? A Switchgear can be described as an apparatus which is used for switching, controlling and protection the electrical equipments and circuits.

Switchgear equipment is essentially concerned with switching and interrupting currents either under normal conditions or abnormal conditions. For example a tumbler switch with ordinary fuse which is used to protect the equipments in homes is the simplest form of switchgear. For higher rating H.R.C. fuses (High rupturing capacity) are used in conjunction with switch may serve the purpose of controlling and protecting the circuit.

However different types of switchgear are used for different capacity of electrical systems. The tumbler switch or H.R.C. fuse cannot be used for High Voltage systems, because their replacements takes time and they are not capable of interrupting large fault currents.

In order to overcome above disadvantage, Circuit Breakers are developed. A Circuit Breaker is switchgear which can open or close electrical circuit under normal as well as abnormal conditions and it can also be operated automatically or manually.

Some of essential features of switchgear

·         High reliability: When fault occurs on any part of power system, the switchgear must operate to isolate the faulty section from the rest of circuit

·         Complete Discrimination: When fault occurs in power system or any section of power system, the switchgear must be able to discriminate between faulty section and healthy section

·         Quick operation: When fault occurs in an power system the switchgear must operate as quickly as possible so that no damage is done to the equipments connected to the same line

·         Provision for manual control: Although switchgear operates automatically, there should be provision provided to operate them manually as well.

·         Provision for instruments: There must be provision for instruments which may be required. They can be ammeters, voltmeters, etc

Following are the types of switchgear equipments

·         Relays

·         Switches

·         Fuses

·         Circuit Breaker

I hope after referring above article you understood what are switchgear and its function. In the next article we will go through one by one the types of switchgear, their types, construction, function, etc.

Types of Circuit Breaker

AIR BREAK CIRCUIT BREAKER

Construction

In this type of Circuit Breaker arc extinction and contact separation takes place in air at atmospheric pressure. Figure (a) shows closed current carrying contacts. As the contacts are opened, arc is drawn between them. The arc core is conducting part of plasma. Ionized air is contained in surrounding medium. By cooling the arc, the diameter of arc core is reduced. The arc is extinguished by lengthening the arc, cooling the arc and splitting the arc. The arc resistance is increased to such an extent that the system voltage can’t maintain the arc and arc gets extinguished.

The last figure illustrates normal arrangement of an air break circuit breaker. This type of breaker is used for medium and low voltages.

There are two sets of contact: Main contact (1) and Arcing contact (2). Main contacts conduct the current in closed position of the breaker. They have low contact resistance and are silver plated. The arcing contacts (2) are hard, heat resistance and are usually of copper alloy. While opening the contact, the main contacts dislodge first. The current is shifted to arcing contacts. The arcing contacts dislodge later and arc is drawn between them (3). This arc is forced upwards b the electromagnetic forces and the thermal action. The arc ends travel along the Arcing horns. The arc moves upwards and is split by splitting plated (5). The arc is extinguished by lengthening, cooling, splitting, etc.  But this Air break Principle is used for DC Circuit Breaker upto 15KV.

Arc Extinction in A.C. Air Break Circuit Breaker

In A.C. Air Break Circuit Breaker, the arc is lengthened, cooled and splitted so as to increase the resistance of the arc. The rapid increase in the arc resistance causes the reduction in the fault current and the fault current does not reach the prospective high value. The arc extinction process is assisted by current zeros and A.C. wave. The voltage drop across  the arc goes on increasing with the increase in arc resistance and at a current zero, when  the recovery voltage across the contacts is less than the arc voltage, the arc gets extinguished.

Arcing Horns

As soon as the arc leaves the vicinity to the contacts it communicates to the pair of run out horns. In doing so outer blow out system is switched on. There blow out coils provide a magnetic field such that the arc footing is subjected to strong magnetic field. We know from the electromagnetic theory that force is experienced by current carrying element of length dl meters, carrying current I amperes and placed in magnetic field B Wb/m² is given by,

                                         

dF=I(dl*B)

By virtue of this force the arc travels upwards and its length increases. The tips of the arc run along the arc runners and come to its extreme point. As the length of arc increases at a particular length the system voltage is unable to maintain the arc and the arc is extinguished. For systems having low inductance the energy      (½ LI² ) is small and arc gets extinguished before reaching the extremity of runners. For high inductance circuits special techniques such as air blast, additional larger acing horns etc are used.

*other types of CB will be posted soon

Fuses

Fuse is a simplest current interrupting devise for protection against excessive currents.Several developments and changes have been made in today's fuse and variety of fuses are available. The first was developed by Edison. Some types of fuses are used for arc Extinguishing that appears when fuse elements melts. Generally fuse elements are classified into two types.

1)Low Voltage fuse
2)High voltage fuse

LOW VOLTAGE FUSE

1)Semi-enclosed rewireable fuse:
This type of rewireable fuse is used where low values of fault current are encountered. It consists of base and fuse carrier. The base of porcelain and carries the fixed contacts to which the incoming and outgoing phase wire are connected. The fuse carrier is also of porcelain and holds the fuse element between its terminals. The fuse carrier can be inserted in or taken out of base when desired. When Fault occurs, the fuse element is blown out and the circuit is interrupted. the fuse carrier is taken out and the blown out fuse is replace by new one. the fuse carrier is then reinserted in the base to restore the supply. This type of fuse has two advantages.
1)It allows us to replace the blown out fuse by a new one
2)It is very cheap

Disadvantages
1)It cannot be used for high amount of fault current
2)It generally operated at lower than its real rating
3)It has not good amount of protection from external factors.

2)High rupturing capacity cartridge fuse:
The drawback of rewireable fuse having capacity to carry low fault current is overcome by this fuse. This type of fuse consists of heating resisting ceramic body having metal end caps to which they are wielded. The space within the body surrounding the element is completely packed with a filling powder. The filling material may be chalk, Plaster of Paris, quartz or marble dust and acts as an arc blowing and cooling medium. So this fuse carries normal current without overheating. When fault current passes, the fuse element  melts and stops current reaching its maximum value. The heat produced during melting of fuse element is used to  vapourise the melted silver element, which reacts with the filling powder and forms a high resistance substance which is used for arc quenching.

Disadvantages
1)Heat produced may sometimes affect the switches and other devices
2)Replacement is to be done after each operation.

Advantages
1)They are reliable and high speed of operation
2)They are cheap and require less maintenance
3)they are capable of clearing low and high fault current , thus they have consistent working.

3)H.R.C. fuse with tripping device
There are some type of H.R.C. cartridge fuse provided with ripping device. the reason for using a tripping devise is when fuse is blown out under fault conditions, the tripping device causes the circuit breaker to operate. The body of fuse is made up off ceramic material with a metallic cap rigidly fixed at each end. These are connected by a number of silver silver fuse elements. At one end is plunger which under fault conditions hits the tripping mechanism of the circuit breaker and causes it to operate. The plunger is electrically connected through a feasible link, chemical charge and a tungsten wire to the other end of cap. When a fault occurs, the silver fuse elements are first blown out and then current is transferred to tungsten wire. The weak link in series with the tungsten wire gets fused and causes the chemical charge to be detonated. This forces the plunger outward to operate circuit breaker.The travel of plunger is so set that it is not ejected from fuse body under fault conditions.

Advantages
1)It is generally capable of dealing with small small fault current, which avoids changing of fuse after getting used
2)If there is fault in any one phase of 3 phase system, the plunger will break open all he contacts of all the three phases.

HIGH VOLTAGE FUSE

Low voltage fuse fail to operate where we are dealing with high current rating. So to overcome this, High voltage fuse is used. They are of following types:

1)Cartridge type

2)Liquid type

3)Metal Clad fuses

1)Cartridge type fuse
This is similar in construction to low voltage cartridge type fuse and some additional features are added. Some design to use fuse elements wound in helical form to avid corona effects at higher voltages. On some of them there are two fuse elements in parallel; one of low resistance silver wire and other of high resistance tungsten wire. Under normal load condition, the low resistance silver wire carries current. When fault occurs, the low resistance silver wire burns and blows out and high resistance tungsten wire reduces the short circuit current and finally opens the circuit.

2)Liquid type
This fuses are filled with carbon tetrachloride and have more number of applications in high voltage system.It consists of glass tube filled with carbon tetrachloride solution and sealed at both the ends with brass caps. The fuse wire is sealed at one end and other end is held by strong phosphorous bronze spiral spring fixed at the other end of glass tube. When the current exceeds the rating of the fuse, the fuse is blown out. As the fuse melts, the spring again attracts part of it through a liquid detector and draws it well into liquid. The small gas generated at the point of  fusion forces some of liquid into the passage through liquid detector and at that point it extinguishes the arc.

3)Metal clad fuses
They are used in order to provide protection to oil circuit breaker. They are used in high voltage system and operate under short circuit conditions approaching their capacity.

Starters

Why starter is needed?  

Basically for a dc motor,

Current = (Applied Voltage - Back EMF) / (Armature resistance)

The back emf is directly proportional to speed and is ZERO at start when the speed is zero.

Hence for a 110 volt dc motor having typical armature resistance 1 ohm,

Starting current = 110/1 = 110 A

The rated current will normally only be around 20 A and hence the starting current is extremely large. This may damage the armature winding.

Hence starters are used to limit the starting current by providing high resistance at start and gradually removing them off as the speed increases.

DC motors are always self starting and they dont need external starting aid. So it's wrong to say dat a starter is used to start a motor

3 POINT STARTER

Generally 3 point starter is used to start shunt type of motor and compound type of motor in industries.

The reason for using starter is the during the starting of motor speed is zero, but the initial Armature resistance and starting resistance are very small so the motor draws high current initially at the time of starting is the voltage equal to rating of motor is applied at the time of starting. it could be around approximately 15 times than current then the current when the motor is working at full load. So this large amount of initial current may damage the other equipments.

The basic principle of starter is to resist the huge amount of the current to flow through the motor.

1)The point L is connected to armature terminal always connected to +ve DC supply and field terminal is connected to -ve DC supply.
2)Point F is connected to field winding and,
3)Point A is connect armature winding of motor through the resistance no. 10  i.e; the point at which motor is ON.

Initially at the time of starting the motor is kept off. The arm of starter is at maximum resistance so no current flows through it. When the arm is moved towards stud no. 1, the field circuit is directly connected to line and full starting resistance is applied in series with armature.the starting current drawn by armature is given by

Ia = E/(Ra+Rst).As the arm is moved towards stud no. 2,3,4,....the starting resistance goes on decreasing. When the arm reaches its maximum position i.e; at stud no. 10,  the resistance becomes nil, then the motor starts working.This is because back emf is developed with the speed to counter supply voltage and slow down armature current.

So question arises whether the handle remains at ON position or returns to OFF position?
There is soft iron piece on the handle.when the field current flows on No voltage coil its acts as electromagnet and holds the iron piece attached to the handle towards it. When the supply fails the electromagnetic properties of NVC disappears and  the handle is returned towards its OFF position due to strong spring connected to handle as shown in fig. When the supply is given the above process is repeated.

What happens when overloading of starter takes place?

This is the point when the Overload Coil comes into picture. When predetermined limit of current is crossed, then the current flowing through Overload coil will produce attractive force which will lift upwards the triangular iron piece. So now the No voltage Coil is shorted and voltage across it reduces to zero and handle returns to OFF position.

Drawback of 3 point started    

1)The NVC and field winding are connected in series. so current in NVC will be low if we reduce field current to apply flux control.
2)If current in below certain limit, the attractive force produced by NVC may not be able to hold in RUN position, thus the handle will return to OFF position and motor will be turned OFF.
3)It is unsuitable for use with variable speed motors. To overcome this 4 Point starter is used.

*4 Point starter will be posted soon

                                   

In Fig. is shown a, four-point starting rheostat, to which four connections are required. This type of rheostat is required by adjustable-speed motors in which the shunt-field current may be so greatly reduced at high speeds that a series holding coil would release the starter handle. In the four-point starter the holding-coil current is independent of motor operating conditions. The four-point starter is suitable for starting a series motor, whereas the three-point starter obviously is not.

In both the three-point starter and the four-point starter, the handle is returned to the "off" position by a spring if the line voltage becomes too low.

Some starters or controllers, like that illustrated in Fig., are intended for adjustable-speed duty and are designed so generously that the starting handle may be left on any control point indefinitely without causing overheating. Such controllers cost more than simple starters and, when such equipment is purchased, it is advisable to specify the type of duty - whether simple starting or combined starting and speed control. By permitting some of the starting resistance to remain in the circuit, it is possible to obtain motor speeds lower than those which may be had by shunt-field control. The efficiency of such control is poor because of the loss of power in the rheostat. Also, the speed regulation of a motor controlled by series resistance in the armature circuit is high (great), and this type of control is thus rendered unsuitable for many purposes.

Parallel Operation of Transformers

When the load current increases above the capacity of one transformer, we can connect one more in parallel instead of replacing the existing transformer with a bigger one.Following are some of the reasons for connecting transformers in parallel.
1)Higher Reliability
2)Better maintenance
3)Its is easy to make addition in existing setup
If a single large transformer is used then its failure will shutoff the installation. The parallel transformers solve this problem and improve reliability. It is possible to repair or maintain the transformers in parallel connection one by one without shutting off the plant. With increase in load current we can easily add new transformer to existing parallel unit.

Conditions to be satisfied

1)The transformers should be connected taking their polarity into consideration such that the net voltage around the loop is zero. If the polarities go wrong then there will be a short circuit.

2)The order in which the phase (RYB) reaches their maximum positive voltage must be same for both transformers, if not then they will be each pair phase will  be short-circuited

3)The voltage ratio of transformers should be the same. This will avoid the no load circulating current

4)The per unit impedance of the paralleled transformers should be approximately equal. The currents delivered by the two transformers  are proportional to their ratings of their per unit impedance are equal. The ratio of equivalent leakage reactance to equivalent resistance should be the same for all the transformers. If they are unequal, then the transformers will operate with different power factors.

Corona

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if applied voltage is low. However, when the applied voltage exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called CORONA.

The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss and radio interference. The higher the voltage is raised, the larger and higher the luminous envelope becomes, and greater are the sound, the power loss and radio noise. If the applied voltage is increased to breakdown value, a flash over will occur between the conductors due to breakdown of air insulation.

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is called as corona.

If the conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter. With D.C. voltage, there is difference int he appearance of two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow.

Corona Formation

In a power system transmission lines are used to carry the power. These transmission lines are separated by certain spacing which is large in comparison to their diameters. In Extra High Voltage system (EHV system ) when potential difference is applied across the power conductors in transmission lines then air medium present between the phases of the power conductors acts as insulator medium however the air surrounding the conductor subjects to electro static stresses. When the potential increases still further then the atoms present around the conductor starts ionize. Then the ions produced in this process repel with each other and attracts towards the conductor at high velocity which intern produces other ions by collision. The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow of violet colour appears together along with hissing noise. This phenomenon is called virtual corona and followed by production of ozone gas which can be detected by the odor. Still further increase in the potential between the power conductors makes the insulating medium present between the power conductors to start conducting and reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon constitutes Corona discharge effect in electrical Transmission lines.

SF6 Circuit Breaker and its maintenence

First of all what is a Circuit Breaker?

A CB is an element which automatically opens circuit under fault conditions in order to prevent damage to the equipment.

Principle of CB:

It consists of two electrodes which are fixed and movable. Whenever Fault occurs the trip coils of the CB get energised and the movable contacts are pulled apart, which will open the circuit. But under normal conditions these contacts remain closed.

SF6 Circuit Breaker

It is basically a suplhur Hexaflouride Circuit Breaker and this SF6  gas is used for arc quenching mechanism. SF6 is electronegative gas and has high tendency to absorb electrons. (arc mechanism will be covered later)

Due to its electronegativity the low arc time constant , the SF6 gas regains its dielectric strength rapidly after final current zero, the rate of rise of dielectric strength is very   high and time constant is very small.

In SF6 CB, the gas is made to flow from high pressure zone to low pressure zone through a convergent-divergent nozzle.The mass flow is a function of nozzle throat diameter, the pressure ratio and time of flow. The nozzle is located such that flow of gas covers the arc. The gas flow attains almost supersonic speed in the divergent portion of the nozzle, thereby the gas takes away the heat from the periphery of the arc, causing reduction in the diameter of arc. Finally the arc diameter becomes almost zero at the current zero and the arc is extinguished. The arc space is filled with fresh SF6 gas and the dielectric strength of the constant space is rapidly recovered due to the electronegativity of the gas.

Image Source: Google Images

Handling Non faulted SF6

The procedures for handling nonfaulted SF6 are well covered in manufacturer’s instruction books. These procedures normally consist of removing the SF6 from the circuit breaker, filtering and storing it in a gas cart as a liquid, and transferring it back to the circuit breaker after the circuit breaker maintenance has been performed.No special dress or precautions are required when handling nonfaulted SF6.

Handling Faulted SF6

Toxicity

Faulted SF6 GAS- Faulted SF6 gas smells like rotten eggs and can cause nausea and minor irritation of the eyes and upper respiratory tract. Normally, faulted SF6 gas is so foul smelling no one can stand exposure long enough at a concentration high enough to cause permanent damage.

Solid Arc Products– Solid arc products are toxic and are a white or off-white, ashlike powder. Contact with the skin may cause an irritation or possible painful fluoride burn. If solid arc products come in contact with the skin, wash immediately with a large amount of water. If water is not available, vacuum off arc products with a vacuum cleaner.

Normally, at least once a year or after every 500 operations, the circuit breaker must be maintained. During

maintenance, the moving parts of the mechanism must be lubricated carefully. The insulating parts are to be wiped

out by a clean and dry cloth. When maintaining, the circuit breaker should be open and high voltage sides must be

grounded. Auxiliary power supply should also be disconnected. On saline areas near seaside, the insulating parts

of the circuit breaker must be carefully cleaned, at least once every two months. If not, the microscopic salt particles

drawn by wind from the sea will create conductive layers on the insulating surfaces and may cause surface flashover.

Before maintenance, first circuit breaker, then isolator should be opened and grounded carefully. The maintenance

of circuit breaker must be done after checking the open position of isolator contacts by eye.