Lightning Phenomena


Lightning Arrester or surge suppressor is the protective device used in every electric substation (small or large) which protects the substation from lightning and other electrical transients. In the next article we will discuss about lightning arrester. Before we discuss about lightning arrester it is important to learn  some basics about the lightning phenomena. After all it is a very interesting subject. In last few centuries many scientists, engineers, philosophers and others have shown considerable interest in studying the lightning phenomena (perhaps due to the curiosity gathered from the childhood). The scientific theories formulated by them differ in some some aspects. However still there is much agreements between the studies carried out. For example in almost all the studies it is mentioned that in most cases the upper part of the cloud is positively charged and the lower part is negatively charged. (The way it is charged or the rise in charge quantity is explained differently by different researchers). We will discuss this in a manner which is well accepted and simple to understand.

See the figure below. Consider the cloud whose upper part is positively charged and lower part is negatively charged. Due to this the portion of earth below the cloud becomes positively charged. As the charge on the cloud increases, so also the charge induced on the ground below the cloud increases. This situation results in rise in the electric field intensity in the space between the cloud and the ground. Finally electric breakdown of the space between the cloud and ground takes place which is seen in the form of an electric discharge.



The sequence of lightning stroke is as follows. As the charge on the lower portion of the cloud increases the electric field increases. This results in ionization of air in the vicinity of area with more charge concentration. Then an ionized path or streamer (stream of charges) develops. This is called as stepped leader(see below). The leader proceeds downward in steps of tens of meters with a speed of about one tenth the speed of light. Several leaders may originate from the cloud at different parts of the clouds.



The ground objects mainly the taller structures also ionize the air in the vicinity of its top and releases a stream of positive charge. That is to say the upward moving leaders usually originates from taller structures (see above).

Finally the downward leader from the cloud may connect the upward moving leader. It results in a ionised conducting path giving rise to heavy current flow through this ionized path. This is called the return stroke. This lightning stroke may carry hundreds of kilo-ampere of current and extremely bright.  This is known as direct lightning stroke. It may hit transmission lines and other electrical equipment developing voltage surge of very high peak value and causing extensive damage to the equipment or catches fire.

Sometimes another form of surge develops in the overhead line. See the figure below in which there are two charged clouds.



The portion of the transmission line below the cloud develops positive charge by electrostatic induction. If the lightning discharge takes place between the clouds then sudden disappearance or rearrangement of charge on the clouds results in instant release of induced charge from the line portion (see figure below). This phenomenon gives rise to travelling voltage waves (due to moving charges)  to both directions of the line (surges are shown side by). This voltage surges hit the electric equipment at line terminating points which may be a substation or generating station. The surges are equally harmful to line insulators. This is the induced voltage surge due to electrostatic induction.



Voltage surges may also develop due to magnetic induction. Due to the high current in the lightning stroke  and/or lines due to surge current, strong magnetic field is created around it. Any conductor experiencing this magnetic field  variation will develop voltage surge.





CT And VT Comparison And Connection


In the last two articles we discussed about the Voltage Transformer(VT) and Current Transformer(CT). Here we will discuss few more facts about the instrument transformers and compare the two. In industry the voltage transformer is also called as Potential transformer(PT). In a power system representation. CT and VT or PT are shown symbolically. Below are the symbols commonly used for VT and CT. Sometimes VT is also represented by two overlapping circles (as in power transformer). Also similar to power transformer, small round dots are used for identification of polarities of the instruments. Proper polarities identification is important for connection of instrument transformers.



Here below we compare some important characteristics of CT and VT
  • A Voltage Transformer(VT) transform high voltage of the primary side to low voltage and Current Transformer(CT) Transform  high primary side current to low current. These low values of voltage and current can be readily used by the measuring and protection instruments.
  • The instrument transformers insulate the low voltage measuring and protection instruments from the high voltage side. It enhances safety for the personnel at the low voltage control and protection side.
  • The instrument transformers make it possible for standardization of instruments and relays etc.
  • VT is connected between the line and ground or between the lines(see fig-B). In high voltage application it is usually connected between the line and ground. A CT is connected in series with the line (fig-B).
  • When energized from the primary side the secondary of CT should never be kept open and secondary terminals should be shorted. The VT secondary should never be shorted. So a fuse is not inserted in the secondary of CT. But a fuse can be inserted in the primary or secondary side of VT.
  • The primary of CT carries the actual current of the Line whose value of current is to be measured or sampled. Hence the primary side of CT is comprised of thick conductor to carry line current and the secondary side has several turns of conductors of thinner cross section. In a VT the primary side voltage is high  so there are large numbers of turns in the primary side of thinner conductor. The secondary side of VT has few turns of conductor of large cross sectional area. The secondary side carries large current for supply to the burden.
  • For a VT the ideal transformer Law, Vp/Np = Vs / Ns is important. For a CT the transformer Law IpNp =IsNs is important. The design of the VT and CT should be such so that these ideal laws are satisfied to good accuracy for the respective instrument transformers. 

CT and PT Connection


You have studied in the school that the ammeter should be connected in series with the load and voltmeter should be connected across the terminals of the load for which the voltage is to be measured. Similarly A CT is connected in series with the load or line and the VT is connected between the line and ground (or between the terminals of load or source).  See the figure below. In the figure is a simple system with an AC source and a load connected by a line. This is the HV system shown in thick line. The connection of CT, VT and other measuring and protection instruments are shown with thinner lines. V and I are the voltmeter and ammeter connected to secondary sides of VT and CT respectively for measurement of voltage between the lines and current I flowing through the line. R is the Relay, one is connected across the secondary of VT and another is in series with CT secondary.  The relay connected in VT is the voltage operated type and the one connected in the secondary of CT is current operated type. The VT is also shown as connected to the pressure coil of the watt meter and the CT is connected to the current coil of the same watt meter. The VT and CT can be connected to many measuring and/or protection devices. But the sum of the burdens of the devices should not exceed the rated burden of the Instruments.



VT and CT are the measuring instruments and the main purpose is to measure the circuit condition or parameters. So the connection of the instrument transformers should not influence or alter the original circuit condition. It follows that the CT is desired to have very little impedance (or resistance) across its terminals. So that the CT in series with the line should not result in any significant voltage drop across its terminals. The current flowing in the secondary of the CT  does not influence the primary side current.  The primary side current is solely determined by the load impedance, source voltage and of course the line parameters.


A voltage transformer is connected between line and  ground. It is desired to have very high impedance. A low impedance results in comparatively large current flow in VT primary and can considerably alters the original circuit condition which is not desired. Otherwise we can say that the voltage transformer should have negligible loading effect on the main circuit. In figure-B, the values of Iz and I should not show any noticeable difference due to connection of VT and CT.

 In fig-C is shown the SLD of the simple system illustrating symbolic connection of CT and VT.







Current Transformer


In the last article we discussed about the Voltage Transformer(VT). Here we will discuss about the Current Transformer (CT). As we know the voltage and current transformers are used in the electric substations to transform high magnitude voltage and current to low magnitude voltage and current suitable for metering and protection purposes. This article require some basic knowledge about the transformer. It is even better if you go through the previous article about voltage transformer.

In the last article we had written two fundamental relationships between the voltage and current of both sides of the ideal transformer. 

For the current transformer the second equation is the main focus, which is reproduced here for convenience.

 The ampere-turns of both sides of the transformer are equal. Otherwise this equation is also called as the conservation of ampere-turns. So for the ideal transformer,

Ip  Np =  I Ns 



The subscripts 'p' and 's' are used for primary and secondary sides of the transformer respectively. Where the symbol I is used for current and 'N' is for number of turns. From the above formula it is easy to guess that for a transformer as Np and Ns are known values which does not change, so the value of secondary current Is is proportional to the primary current Ip, which is desired for accurate measurement of primary current. This is only true for ideal transformer. In case of actual transformer the primary and secondary side currents are not proportional. (see the equivalent circuit of the transformer in last article)

In an actual transformer a small part of the primary current is used as exciting current of the transformer core. So Ip and Is are no more proportional. Hence in the design of the current transformer the main aim is that the excitation current of the current transformer should be low.

Burden And Error


As said above, error is introduced in the measurement of the secondary current. The error happens both in the magnitude and phase angle of the current. The Error in magnitude is said as Current Ratio Error and the error of phase angle is called as Phase Error. the Curret Ratio Error in percentage is given as:

Current Ratio Error = (Kn * Is - Ip ) * 100 / Ip

Here Ip and Is are the actual primary and secondary currents (rms). 

Where as Kn = Ipr / Isr

Ipr and Isr are the rated primary and secondary currents and Kn is the rated transformation ratio.

Similar to the voltage transformer the load on the secondary of the current transformer is called as burden. The burden is either expressed in volt-amperes (VA) or in Ohms. The burden of the CT is the  sum of the burdens due to all  the equipment connected to the CT plus the burden due to the connecting cables.

The CT secondary current is usually rated at 5 Amp or 1 Amp. The secondary current rating should be chosen judiciously according to the application. Suppose we choose a CT of secondary current 5A. If the equipment for connection to the secondary of this CT requires long cables then the burden due to this connecting cables may be a considerable proportion of the total burden.

Burden Due to Cable = IR

R is the total resistance of connecting cables. I is the secondary current of CT. From the above you can have a feel about the value of burden due to cable for both the cases of 5A and 1A secondary rating.

So for the cases where long connecting leads are required then smaller current rating of 1A should be chosen.


Accuracy of current transformer is defined by different standards . The table below is a part of the IEC standard. The full table may be obtained from their web site.


Current Transformer Accuracy Table
Current Transformer Accuracy Classes (As IEC 60044-1)
(Partial Table)

Accuracy Class
Percent Rated current
Current Ratio Error (%)
Phase Error (Minutes)
Application
0.1
5
+0.4
+15
Precise Measurement
0.1
20
+0.20
+8
Precise Measurement
0.1
100
+0.1
+5
Precise Measurement
0.1
120
+0.1
+5
Precise Measurement
0.2
5
+0.75
+30
Measurement
0.2
20
+0.35
+15
Measurement
0.2
100
+0.2
+10
Measurement
0.2
120
+0.2
+10
Measurement



In the table if the secondary current phasor leads the primary current phasor then the phase difference is positive, so the phase error is positive otherwise the error is negative. If you have drawn the phasor diagram for a basic transformer so that the angle between Ip and Is is nearly 180 degrees, then before doing the comparison the secondary phasor should be reversed. (if the CT is an ideal one then the angle between the two phasors should be zero). The above accuracy class table is valid for the burden between 25% to 100% of rated burden.

Construction


From the constructional point of view the current transformer is broadly available in two types. These two are Ring Type and Wound Core type.

The Wound Primary type has the primary winding of one or more turns over the core of the CT. The Window or Ring type does not have a primary winding. Of course both the types have a secondary winding.  The ring or window type has an opening in the center. The primary side is the single conductor which can pass through the opening (see figure below). 



Another variant of ring type is available which is called as bar type. Here a conductor bar is already occupying the center of the core and so a part of the CT. The terminals of the bar are brought out to be connected in series with the power circuit of which the current to be measured(or for protection). One more variation of ring type CT is the split core type. A spring mechanism allows the core to split and the primary conductor is allowed to the center of the core. Then the core is closed. This type of CT is mainly used for current measurement of low voltage distribution maintenance work below 450 volts.

The ring and bar types are mainly used for medium and low voltage CT where as wound primary type is mainly used for high and extra high voltage application.

The HV current transformers are designed as live tank type or dead tank type. The basic design of a CT for HV use is illustrated in figures below. The two types of CT mostly used in high voltage application are Hair Pin type and Top Core type. The Hair Pin type is so named as its primary side conductor resembles a hair pin.The Hair Pin type is mechanically more stable and robust in comparison to Top Core type. In HV application the core of the Hair Pin type is usually insulated by oil or SF6 (sulphur Hexa Fluoride) gas, which resides in a steel tank at the bottom position of the CT module.



The primary conductor in a Top Core type CT is straight passing through the core. The core is positioned at the top of the CT inside a box. See the above figures illustrating the relative position of different parts for both types of high voltage CT. Also in the figure below is shown a sketch of the Top Core type CT. It should be noted that for simplicity the secondary winding is shown as consisting of 2 or 3 turns concentrated at one place. But the actual CT whose secondary winding is comprised of several turns and the winding turns are evenly distributed along the core.

The CT is connected in series with the circuit of which the current is to be measured. When on the live system, the secondary side of the CT should never be left open. If the secondary is not used then the terminals of the secondary are shorted. If the secondary is left open then excessive high voltage is induced in the secondary which is harmful to the personnel and also to the equipment insulation. Also the CT should not be operated with a high resistance burden. The burden here is the total burden due to all the equipments connected to the CT plus the burden due to the cable.



The requirements of CT for the purpose of metering and protection are different. While for the metering purpose the accuracy within certain limits are very important where as in case of protection use, the CT should be able to give a reasonably proportionate secondary current for a variation of primary current which is many times the rated primary current.   The Accuracy Load Factor (A.L.F)  is the ratio of primary current upto which the CT gives reasonably proportionate secondary current  to the rated current. A.L.F is a number which gives the idea that upto how many times of the rated primary current, the CT gives reasonably proportionate secondary current. It is very essential for protection purposes, which is the requirement at faulted system condition with large fault current.

In this article and the previous article we have covered two very important equipment of a electric substation. Effort is made to present the information in simple and accurate ways. For the actual use of the CT and PT one should consult the manufacturer instruction sheet, guidelines and the relevant international and local standards. In the next article we will discuss little more and compare CT and PT and their connection.

Voltage Transformer


Voltage Transformer and current Transformer are known as Instrument Transformer. They are used in the substation to transform high magnitude voltage and current to low magnitude voltage and current suitable for metering and protection purposes.

While the main purpose of the instrument transformer is metering and protection it also isolate the high voltage side from the low voltage side comprised of  measurement and protection devices and circuits. Before proceeding further one should have some basic knowledge about working of the transformer. Here in this article we discuss about the voltage transformer (also called as potential transformer) and in the next article we will discuss about current transformer. The voltage transformers are broadly of two types. These are inductive VT and Capacitive VT (CVT). Let us first consider the inductive (electromagnetic) Voltave Transformer.

We Know the two fundamental laws of the inductive transformer.

For an ideal transformer,

V/ Np =  V/ Ns 

Ip  Np =  I Ns 

Subscripts 'p' and 's' are used for primary and secondary sides of the transformer. N is the number of turns of the respective side of the transformer.



The voltage and current transformers are used for measuring or protection purpose. Hence  in the ideal case we desire to get the value of secondary voltage which is proportionate to the primary voltage. The voltage transformer (potential transformer) is designed to closely follow the formula VNp =  VNs  for a specified range of operation. 

The equivalent circuit of an actual voltage transformer is shown in Fig-B.  Rand Lp are primary side resistance and leakage reactance of  transformer. Rand Ls are for the secondary of transformer. Rand Lm are the core loss component and magnetising reactance component respectively.  Error is mainly introduced in the measurement of voltage and phase angle due to these parameters of transformer.



 In comparison to current transformer the voltage transformers operate at a relatively higher point of the operating curve. In the design process care is taken to limit the exitation current otherwise the increased exciting current will result  in excessive voltage drop in the series impedances, so the error is increased. 

The inductive voltage transformer is constructed similar to power transformer. The secondary(low voltage) side winding has few turns wound over the magnetic core and the primary (high voltage) side winding is comprised of several turns wound over the primary winding.  The cross sectional area of the secondary side conductor is considerably more than the primary side conductor.  The secondary side voltage adopted is usually 100 volt or 110 volt.

A sketch of voltage transformer is shown in Fig-C. The porcelain insulator provide required creepage distance for HV terminal from ground. The tank made from galvanized steel filled with oil contains the magnetic core wound with primary and secondary windings of VT.  In a voltage transformer the core size is comparatively more so that a low flux is maintained at operating point.




Burden and Error


The instrument transformers are classified according to the allowed percentage error and burden. The load on the secondary side of the voltage transformer is called as burden(For instrument transformers burden terminology is used instead of load on the secondary side).The rated burden is specified in voltampere or VA. The Total burden of all the instruments connected to the secondary of the voltage transformer (VT) should be less than the rated burden. For example the VT secondary may be connected to a voltmeter, a watt meter, Integrating meter, a synchroscope and some relays. The sum of the burdens of all these equipments should be less than the rated burden of the VT. More over if the conductor lead used for connecting to these instruments is very long, then the  burden due to this long lead should also be added to the burdens of all the equipmets connected to the secondary of the Voltage Transformer. The burden of the VT can also be specified by impedance value in Ohm.

The voltage transformers has a specified rated transformation ratio. If kn is the rated transformation ratio then voltage error in percentage is given as,


Voltage Error = ( kn * Vs  Vp ) *100 /  V 


V and  Vp  are the actual primary and secondary voltage. 

And Kn is the ratio of rated primary voltage to rated secondary voltage.  

The  Accuracy class of voltage transformer (VT & CVT) is defined by the IEC. The table below display the limits specified  for the accuracy classes.

VT accuracy Table
Voltage Transformer Accuracy Classes (As IEC 60044-2)

Accuracy Class
Voltage Error (%)
Phase Error (Minutes)
Application
0.1
+0.1
+5
Precise Measurement
0.2
+0.2
+10
Measurement
0.5
+0.5
+20
Measurement
1.0
+1.0
+40
Measurement
3.0
+3.0
---
Measurement
3P
+3.0
+120
Protection
6P
+6.0
+240
Protection

(Phase angle Error expressed in Minutes. One degree = 60 minutes)

The protection VTs are less accurate than the metering VTs. For revenue metering purposes the VT with accuracy class 0.2 may be preferred. For indicating meters less accuracy class like 1.0 may be chosen.

For the metering VTs the above accuracy of VT should be valid for voltage range between 80% to 120% of the rated voltage. For the protection VTs the above accuracy of VT should be valid for voltage range from 5% to  Vf times the rated voltage.  Vf  is the voltage factor. Vhas been defined by IEC. Vf is equal to 1.5 for solidly earthed system and 1.9 for the system which is not solidly earthed(See IEC standard).
For both metering and protection VTs, the above accuracy of VT should be valid for the burden between 25% to 100%. of rated burden.

Capacitor Voltage Transformer (CVT)


The above described inductive voltage transformer is usually economical for system voltage rating upto 132 kV. For higher system voltage at Extra High Voltage (EHV) and Ultra High Voltage(UHV), Capacitor Voltage Transformers (CVT) are used. At system voltage above 38 kV the inductive VT is not cost effective. The CVT is basically comprised of a capacitor voltage divider (see figure below) and an inductive Voltage transformer (as described above). The tapped voltage from the last unit of capacitor voltage divider is fed as input to the inductive VT. By using the capacitor voltage divider the system voltage is reduced to a voltage level suitable as input to the transformer.



As the circuit is capacitive a reactor L is connected in the primary so that the sum of the reactance L and the leakage reactance of the transformer compensate the capacitive effect at power frequency.

The phenomon of ferroresonance considerably influences the design of CVT. Under the conditions of various network disturbances or fault conditions the divider capacitor and inductor in the CVT form a series tuned resonating circuit. In resonance the magnetic circuit may saturate and overheat the transformer. It is necessary to damp out ferroresonance in CVT. So the CVTs are equipped with ferroresonance damping circuit as shown in the figure above.


Air Blast Circuit Breaker (ABCB)


We have so far discussed three main types of circuit breakers. These are Vacuum Circuit Breaker, Gas Circuit Breaker or SF6  Circuit Breaker (GCB) and Oil Circuit Breaker. The other type of circuit breaker that we discuss here is Air Blast Circuit Breaker(ABCB). This type of breakers are also becoming obsolete. Once Air Blast type of breakers were preferred in Extra High Voltage substations. Now it is difficult to find new HV/EHV substations equipped with Air Blast Circuit Breakers.

One should not be confused between Air Circuit Breaker and Air Blast Circuit Breaker. Air Circuit Breakers are usually used in low voltage applications below 450 volts. You can today find these in Distribution Panels (below 450 volts). Air Blast Circuit Breakers are high capacity breakers and can be seen in old substations mainly above 132 kV. The working principle of these two circuit breakers are quite different. Here we will only discuss the working of ABCB.

In Air Blast Circuit Breaker, air at high pressure is blast upon the arc formed between the contacts. The air blast blows away the ionized air between the contacts.

See the Sketches (Figs-A and B) illustrating the arc extinction process of the axial blast type breaker . The contacts are in closed position by spring pressure. For opening the contacts. Air at high pressure from the air receiver (Fig-C) is blasted to the interruption chamber. This pressure exceeds the spring pressure and pushes the moving contact away from the fixed contact. This opens the contacts and air at high pressure passes through the nozzle and port to the atmosphere. This axial flow of air at high speed extinguishes the arc within 2 or 3 cycles of current wave and ionized gas is blown away.  Then the port is closed by the moving contact arm(Fig-B)  and the space between the contacts is filled with fresh air at high pressure. This enables the breaker to withstand high Transient recovery Voltage (TRV). Compare Fig-A with Fig-B. In Fig-B the arc is extinguished and spring is in compressed state.



To close the contacts, a valve arrangement lets the air from the chamber to pass to the outside atmosphere. This makes the spring pressure to close the fixed and moving contacts.

Some main advantages of the Air Blast Circuit Breaker(ABCB) are:
  • Arc extinction is very fast. Hence it is suitable for frequent opening and closing operation.
  • Due to refilling of separated contacts space by fresh air at high pressure,  the separation requirement between the contacts is quite less in comparison to OCB. This makes the size of the breaker smaller.
  • The ionized gas flushed out to the atmosphere. Hence unlike OCB here the arc quenching medium does not deteriorate with time. This eliminates some maintenance burden.
  • It is non-inflammable.
  • Finally one important advantage is that in ABCB the arc quenching depends on the high pressure air which is obtained from a compressor, an external source. So in case of ABCB the arc extinction or arcing time does not depends upon the arc current. (In case of OCB the arcing time depends on the current to be interrupted).
  • The breaker breaking capacity depends upon the external source, the high pressure air.
The Air Blast Circuit Breakers has some disadvantages. The important one is that  Air Blast Circuit Breakers  require a compressor plant (not shown in Fig-C) which requires regular maintenance. Hence ABCB is not economical for low voltage applications. There are other issues like current chopping and restriking voltage which requires to be handled by proper design and damping mechanism.



In last few articles we have discussed the working principles of all the major types of breakers used in High Voltage and Extra High Voltage Substations. Perhaps this is enough in developing some basic concepts on an important substation equipment like Circuit Breaker. In subsequent articles we will discuss some other equipment used in HV/EHV substations.


Oil Circuit Breaker


We already discussed Vacuum Circuit Breaker and SF6 Circuit Breaker. In modern power systems these two types of circuit breakers are mainly used for high voltage application. While vacuum breakers are mainly used for voltage upto 38 kV, SF6 breakers are used starting from distribution voltage at 11 kV upto 765 kV and 1200 kV level.  Although the use of oil breaker has reduced very much one can still find oil CB in many installations. So I liked to write a little about oil circuit breaker in one article.

Oil Circuit Breakers (OCB) can be categorised into two types. One is Bulk Oil Circuit Breaker (BOCB) and the other type is Minimum Oil Circuit Breaker (MOCB). MOCB type is also called as Low Oil Circuit Breaker.

Bulk Oil Circuit Breaker (BOCB)

The Bulk Oil CB design is very simple (Fig-A). In Fig-A the arc control device between the fixed and moving contacts is not shown, so making the sketch even simpler. This type of circuit breaker uses a steel tank containing oil and the contacts are immersed in the oil. The steel tank is earthed (dead tank type). In this type construction the oil requirement is more as the oil is required to provide insulation to the contacts from the steel tank and insulation between the contacts(in open state). The oil also serves as the medium for extinguishing the arc formed when the moving contact separates from fixed contact. When the contacts separate, arc is formed between the contacts. The arc gives rise to formation of gas in the oil which initiates oil circulation. This phenomena helps in extinguishing the arc so breaking the circuit. For higher voltage this very simple principle cannot be much effective. So an arc control device is usually used to facilitate arc extinction process.



The BOCB is available as single tank type or three tank type. Usually for lower voltage use, below 38 kV, single tank type is adopted with barrier between the phases.  For higher voltage application three separate tanks are used.

Minimum Oil Circuit Breaker

If you visit an old substation, having BOCB installed, you immediately recognise the oversize Circuit Breaker. As explained above, BOCB is large in size and requires more space. 

Minimum Oil Circuit Breakers (MOCB) require less oil as the purpose here is only to extinguish the arc and not for providing insulation to the contact. Arc interruption takes place inside the Interrupter. The whole system is placed inside the porcelain housing. Because of this insulating porcelain the insulation requirement of contacts is reduced very much. This is the reason of its smaller size. As such the MOCBs are of  live tank outdoor type design and mainly used for voltage levels above 38 kV.

The oil circuit breakers have some severe disadvantages. The main disadvantage is that the OCB can explode causing harm to the personnel and other equipment of the system. The tank type design is very bulky so making it difficult for transportation and handling and requires more space. The OCB requires more maintenance in comparison to vacuum and SF6 breakers. Irrespective of these few disadvantages the OCBs are not going to vanish within few years.

  

SF6 Circuit Breaker Working Principle



At this point we are aware that the medium in which arc extinction of the circuit breaker takes place greatly influences the  important characteristics and life of the circuit breaker. In the last article the working of a vacuum circuit breaker was illustrated. We already know that the use of vacuum circuit breaker is mainly restricted to  system voltage below 38 kV. The characteristics of vacuum as medium and cost of the vacuum CB does not makes it suitable for voltage exceeding 38 kV. In the past for higher transmission voltage Oil Circuit Breaker (OCB) and Air Blast Circuit Breaker (ABCB) were used. These days for higher transmission voltage levels  SF Circuit Breakers are largely used. OCB and ABCB have almost become obsolete.  In fact in many installations SF6  CB is used for lower voltages  like 11 kV, 6 kV etc..

Sulphur Hexafluoride symbolically written as SF is a gas which satisfy the requirements of an ideal arc interrupting medium. So SF6  is extensively used these days as an arc interrupting medium in circuit breakers ranging from 3 kv  upto 765 kv class. In addition to this SF6 is used in many electrical equipments for insulation. Here first we discuss in brief, some of the essential properties of  SF6 which is the reason of it's extensive use in circuit breakers

  • SF6 gas has high dielectric strength which is the most important quality of a material for use in electrical equipments and in particular for breaker it is one of the most desired properties. Moreover it has high Rate of Rise of dielectric strength after arc extinction. This characteristics is very much sought for a circuit breaker to avoid restriking.
  • SF6 is colour less, odour less and non toxic gas.
  • SF6  is an inert gas. So in normal operating condition the metallic parts in contact with the gas are not corroded. This ensures the life of the breaker and reduces the need for maintenance.
  • SF6 has high thermal conductivity which means the heat dissipation capacity is more. This implies greater current carrying capacity when surrounded by SF6 .
  • The gas is quite stable. However it disintegrates to other fluorides of Sulphur in the presence of arc. but after the extinction of the arc the SF6  gas is reformed from the decomposition.
  • SF6 being non-flammable so there is no risk of fire hazard and explosion. 
The construction and working principles of SF6 circuit breaker varies from manufacturer to manufacturer. In the past double pressure type of SF6 breakers were used. Now these are obsolete. Another type of SF6 breaker design is the self blast type, which is usually used for medium transmission voltage. The Puffer type SF6 breakers of single pressure type are the most favoured types prevalent in power industry.  Here the working principle of Puffer type breaker is illustrated (Fig-A).




As illustrated in the figure the breaker has a cylinder and piston arrangement. Here the piston is fixed but the cylinder is movable. The cylinder is tied to the moving contact so that for opening the breaker the cylinder along with the moving contact moves away from the fixed contact (Fig-A(b)). But due to the presence of fixed piston the SF6 gas inside the cylinder is compressed. The compressed  SF6 gas flows through the nozzle and over the electric arc in  axial direction. Due to heat convection and radiation the arc radius reduces gradually and the arc is finally extinguished at current zero. The dielectric strength of the medium between the separated contacts increases rapidly and restored quickly as fresh SF6 gas fills the space. While arc quenching, small quantity of SF6 gas is broken down to some other fluorides of sulphur which mostly recombine to form  SF6  again. A filter is also suitably placed in the interrupter to absorb the remaining decomposed byproduct.

The gas pressure inside the cylinder is maintained at around 5 kgf per sq. cm. At higher pressure the dielectric strength of the gas increases. But at higher pressure the SF6 gas liquify at higher temperature which is undesired. So heater is required to be arranged for automatic control of the temperature for circuit breakers where higher pressure is utilised. If the SF6 gas will liquify then it loses the ability to quench the arc.



Like vacuum breaker, SF6 breakers are also available in modular design form so that two modules connected in series can be used for higher voltage levels. SF6 breakers are available as both live tank and dead tank types. In Fig-B above a live tank outdoor type 400 kV SF6 breaker is shown.




Vacuum Circuit Breaker



The Vacuum Circuit Breakers (VCB) are particularly advantageous for use in the voltage range 3 kV to 38 kV. In the Vacuum Circuit Breaker the arc interruption takes place in vacuum in the interrupter. The pressure inside the vacuum interrupter is maintained below 10-4 torr. At this low pressure very few molecules are available inside the interrupter chamber. This is one desired characteristic of the interrupting medium for more efficient arc quenching.

For opening the circuit breaker, the operating mechanism separates the moving contact from the fixed contact inside the interrupter. Just at the point of contact separation, a very small amount of metal vaporizes from contact tip and arc is drawn between the contacts. Current flows between the contacts through this arc. Due to the sinusoidal nature of the AC current, the current after reaching the maximum value decreases so reducing the vapour emission. Near zero value of the sinusoidal current wave the arc is extinguished. The metal vapour is deposited on the condensing shield  (see Fig-A). The space inside the interrupter being high vacuum, very little ions are available between the electrodes/contacts. So after arc extinction the space between the contacts regains dielectric strength very rapidly which is the most desired characteristics of the arc quenching medium. Due to the rapid regaining of dielectric strength of vacuum inside the interrupter the re-striking does not takes place. In the figure below is shown the main constructional features of a Vacuum Circuit Breaker (VCB).


The vacuum condensing shield is used so that the metallic vapour does not condenses on the enclosure glass. In the absence of the shield the metallic vapour condenses on the glass and gradually the glass becomes conducting, so that the insulation between the moving and fixed contacts is lost in the open condition of the breaker. The metallic bellow makes it possible to maintain vacuum inside the interrupter chamber while allowing the movement of moving contact for separation from the fixed contact. One side of the bellows is welded to the moving contact stem as shown while the other side is welded to the interrupter end plate. The contact surface is so designed that the arc between the contacts diffuse. The arc spread to the sides of the contact surfaces. Diffusion of arc reduces its strength hence the arc quenching is facilitated. The main requirements of the contact material is, very high electrical and thermal conductivity, low contact resistance and high melting point.

Advantages of VCB
  • The vacuum interrupters have long life. 
  • Unlike oil CB (OCB) or air blast CB (ABCB), the explosion of VCB is avoided. This enhances the safety of the operating personnel. 
  • No fire hazard.
  • The vacuum CB is fast in operation so ideal for fault clearing. VCB is suitable for repeated operation. 
  • Vacuum circuit breakers are almost maintenance free. 
  • Due to the rapid gain of dielectric strength of vacuum interrupter, the separation required between the moving contact from fixed contact is of the order of few millimetre. This makes the VCB compact.
  • VCB is light weight.
  • No exhaust of gas to the atmosphere.
  • Quiet operation.
Disadvantages of VCB
  • The main disadvantage of VCB is that it is uneconomical for use of VCB at voltages exceeding 38 kV. The cost of the breaker becomes prohibitive at higher voltages. This is due to the fact that at high voltages (above 38 kV) more than two numbers of interrupters are required to be connected in series.
  • Advance technology is used for production of vacuum interrupters. 
More over the vacuum interrupters production is uneconomical if produced in small quantities.Vacuum interrupters are used in metal clad switchgear and also in porcelain housed circuit breakers.
Vacuum interrupters have been successfully used in some countries for circuit breaker rating above 132 kV. The interrupters of a three phase vacuum CB is shown below.