Report on Building Aspects of Electric Substation

Report on Building Aspects of Electric Substation


A substation is a component of an electricity transmission or distribution system where voltage is transformed  from  high   to   low, or  the  reverse,  using   transformers.  A  transmission   substation transforms the voltage   to a level suitable for transporting electric power over long distances. This is   to  minimize capital   and   operating   costs   of the  system. Once it  is transported close to where it Is   needed,   a   distribution  substation transforms the voltage to a  level suitable   for   the  distribution system. So the   assembly  of  apparatus   used  to  change  some  characteristic of  electric  supply   is called a substation In   a substation   using   step   up   and step down   transformer change   AC voltages   from   one   level   to

Another,   change   AC   to   DC   or   DC   to   AC.   A   substation   may   have   one   or   more   relates transformers, many protective equipment and switches.

Substation   is important   part of   power   transmission   and   distribution   system.   Substations   are   the most critical part of any electrical supply grid. A failure of a single piece of substation equipment

can cause   a   total   grid   collapse   which   may   take   days   or even   longer   to   rectify.   The   continuity   of power supply   depends   on successful   operation   of substations. It   is therefore essential   to   exercise

Extreme care while designing and building a substation. Specific functions of substation are-

Power transformer.

Local network   for Connection point.

Switchyard – Bus bars, circuit breakers, disconnections.

Measuring point for control center – Potential and current transformers.

Fuses and other protection device.

Classification of substations:

There are the two most important ways of classifying a substation. According to

1.   Production requirement

2.   Constructional features

According to Production requirement:

A substation   may   be   called   upon to change   voltage   level   or   improve power factor   or   convert ac power   into   dc   power   etc.   According   to   service   requirement      11kV  substation is

Transformer   substation.   In   this   substations   using   power   transformer   changes   voltage   level   of electric supply.

According to constructional features:

A substation   has   many   components   (e.g.   Circuit   breaker, switches, fuses,   instrument   etc.)   Which must   be   housed   properly   to   ensure   continuous   and   reliable   service.   According   to   constructional features the substation is outdoor type Substation.

The outdoor equipment   is   installed   under   the sky.   Outdoor   type   substations   should   be   in   fenced enclosures or located in special-purpose buildings. Indoor   substations are usually found   in   urban

areas   to   reduce   the   noise   from   the   transformers,   for   reasons   of   appearance,   or   to   protect switchgear from extreme climate or pollution conditions                 .

11/.440 kV Substation Arrangement

The   arrangement   of   substations   can   be   done   in   many   ways.   However   the   main   sectors   of arranging the substations are –

 At load center: Where   voltage is   getting down         11kV   to 400volts using transformer   and   this is near to be load center.

Substation Layout

a) Principle of Substation Layouts

Substation   layout   consists   essentially   in   arranging   a   number   of   switch gear   components   in   an ordered pattern governed   by their   function and rules of spatial separation.

b) Spatial Separation

i.    Earth Clearance:   This is   the   clearance between   live parts   and earthed   structures, walls, screens and ground.

ii.   Phase Clearance: This is the clearance between live parts of different phases.

iii. Isolating Distance: This is the clearance between the terminals of an isolator and the connections.

iv. Section Clearance: This is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works

c) Separation of maintenance zones

Two   methods   are   available   for   separating   equipment   in   a   maintenance   zone   that   has   been isolated and made dead.

i. The provision of a section clearance

ii. Use of an intervening earthed barrier

The choice between   the   two methods   depends   on   the   voltage and whether   horizontal or   vertical clearances are involved.

Equipment   Function


The incoming and outgoing lines are connected to the   bus- bars.

  • Isolator  Disconnect   a   part   of   the   system   for   general   maintenance  and repair under no load condition for safety.
  • Earthling Switch
  • Discharge the over voltage to earth.
  • Circuit Breaker Which   can   automatic   open   or   close   a   circuit   under   normal   as well as fault condition.
  • Lightning Arrestor   discharge   lightning   over   voltages   and   switching   over voltages to earth.
  • Current Transformer
  • Step down current to know the ratio for   control and protection. Voltage Transformer Step down voltage to know the ratio for control and protection.
  • Series Reactors
  • Reduce the short circuit current or starting current.
  • Line Trap   Prevent high frequency signals during low loads.
  • Shunt capacitors Provide   compensation   to   reactive   loads   of   lagging   power
  • Factors.
  • Shunt Reactor in EHV
  • substations
  • To provide reactive power during low loads.
  • Neutral Grounding Resistor Limit the earth current.

Functions of a Substation

1 – Supply of required electrical power.

2 – Maximum possible coverage o f the supply network.

3 – Maximum security of supply.

4 – Shortest possible fault-duration.

5 – Optimum efficiency of plants and the network.

6 – Supply of electrical power within targeted frequency limits (49.5 Hz and50.5 Hz).

7 – Supply of electrical power within specified voltage limits.

8 – Supply of electrical energy to the consumers at the lowest cost.

Elements of a Substation

Substations   have   one   or   more   transformers,   switching   and   control   equipment.   In   a   substation, circuits breakers are used to interrupt any short-circuit or overload currents that may occur on the

network.   Substations   do   not   usually   have   generators,   although   a   power   plant   may   have   a

substation   nearby.   Other   devices   such   as   power   factor   correction   capacitors,   synchronizer   and voltage   regulators   may   also be located   at a substation.   The   main   equipments   of   a   substation are shown –

11/.440 kV   Substation equipments details

Transmission line   set giving   the   rated voltage   level   up to 11   kV.   This   11 kV lines are connected to the 7MVA transformer via 33 kV bus bars is further connected to LT switchgear.

The   equipment   required   for   a   transformer   Sub- Station   depends   upon   the   type   of   Sub-Station, Service   requirement   and   the degree   of   protection   desired.    11kV Sub-Station      has   the   following major equipments.


      7MVA 33/11Kv Main Transformer,

   4 MVA.3 MVA.2 MVA.&1.5 MVA

2.   Lightning   arrestor

3.   Isolator   and Earth switches

4.   Current Transformer

5.   Potential Transformer

6.   Duplicate type bus bar

7.   Insulators

8.   PFI Plant

9.   LT Switchgear –5000Amps, 4000Amps,3000Amps,2000Amps.1500Amps.all LT Switch Gear are Various ACB


Faraday’s law of induction, which states that:

The induced electromotive force (EMF) in an y   closed circuit is equal to  the e time rate of change of the magnetic   flux through the circuit. Or alternatively:

The EMF generated is proportional to the rate of change of the magnetic flux.

where   Vs is   the   instantaneous   voltage,   Ns is   the   number   of   turns   in   the   secondary y   coil   and

equals   the   magnetic   flux   through   one   turn    of   the   coil.   If   the   turns   of   the   coil   are   oriented perpendicular   to   the   magnetic   field   lines,   the   flux   is   the   product   of   the   magnetic   flux    density   B and   the area A through which it cuts. The area is constant, being equal to   the cross-sectional area

of   the   transformer   core,   whereas   the   magnetic   field   varies with   time   according   to   the   excitation of   the   primary.   Since   the   same   magnetic   flux    passes   through   both   the   primary   and   secondary coils in an ideal transformer, the instantaneous voltage across the primary y   winding equals

Electrical power is   transmitted from   the primary   circuit   to the secondary circuit. The transformer

is   perfectly   efficient;   all   the   incoming   energy   is   transformed   from   the   primary   cir cuit   to   the magnetic   field   and   into   the   secondary   circuit.   If   this   condition   is   met,   the   incoming   electric power must equal the outgoing power.

Transformers   normally   have   high   efficiency   more   then   95%,   so   this   formula   is   a   reasonable approximation.   If the   voltage   is increased, then   the   current   is decreased   by   the same   factor. The

Impedance in one   circuit is transformed by the square of th e turn’s ratio.

Transformer E MF equation

If   the   flux   in   the   core   is   purely   sinusoidal,   the   relationship   for   either   winding   between   its   rms voltage   Erm of   the   winding , and the supply frequency   f, number of turns   N,   core   cross-sectional

area a and peak magnetic flux density B If   the   flux   does   not   contain   even   harmonics   the   following   equation   can   be   used   for   half-cycle

average voltage E

a  of any wave shape:

Transformer   ratios:     

The   voltage   ratio   of   a   constant-voltage   transformer,   i.e.,   the   ratio   of primary   to   secondary voltage,   depends   primarily   upon   the   ratio of   the   primary to   the   secondary turns.   The   voltage   ratio   will   vary   slightly   with   the   amount   and   power   factor   of   the   load.   For general work the voltage   ratio   can be taken as equal to the   turn ratio of the windings. The current

ratio   of   a   constant-voltage   transformer   will   be   approximately   equal   to   the   inverse   ratio   of   the turns in the two windings

The   regulation   of   a transformer     is   the change   in   secondary voltage from no   load   to   full load. It is generally expressed as a percentage of the full-load secondary voltage.

The   regulation   depends   upon   the   design   of    the   transformer   and   the   power   factor   of   the   lo ad. Although with a   non inductive   load   such as incandescent lamps, the regulation of transformers is

within about 3   percent,   with   an inductive load   the   drop in voltage between   no load and full load

increases   to   possibly   about   5   percent.   If   the   motor   load   is   large   and   fluctuating   and   close   lamp regulation is important, it is desirable to use separate transformers for the motors.

The   efficiency   of a   transformer     is, as   with   any   other   device, the   ratio of   the   output to   input or, in   other   words,   the   ratio   of   the   output   to   the   output   plus   the   losses.   As   a   formula   it   can   be expressed thus:

The copper loss  of a transformer is determined by the resistances of the high-tension and low-

tension   windings   and   of   the   leads.   It   is   equal   to   the   sum   of   the   watts   of   I   2R   losses   in   these components at the load for which it is desired to compute the efficiency.

The   iron   loss   of   a transformer   is   equal   to   the   sum   of   the   losses   in the   iron   core.   These   losses consist   of   eddy-   or   Foucault-current losses   and   hysteretic   losses. Eddy-current   losses   are due   to

currents generated   by the alternating   flux circulating   within   each lamination   composing the core,

and   they   are   minimized   by   using   thin   laminations   and   by   insulating   adjacent   laminations   with insulating   varnish.   Hysteretic   losses   are   due   to   the   power   required   to   reverse   the   magnetism   of the iron core at each   alternation and   are   determined by the amount and the grade of iron   used for

the laminations for the core.

Transformer ratings.  Transformers   are   rated   at   their   kilovolt-ampere   (kVA)  outputs. If   the load to be supplied   by a transformer is   at 100   percent power factor (pf), the kilowatt (kW) output

will   be   the   same   as the   kilovolt-ampere   (kVA) output.If   the load   has a lesser power facto r, the kW  output  will  be less  than   the  kVA  output  proportionally as the load power   factor   is   less than

100 percent.

Phase Transformer   Connection Construction:

A three phase transformer is constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure which is then filled with dielectric oil. The dielectric oil performs several fun ctions. Since it is a dielectric,   a nonconductor of electricity, it provides electrical insulation between the windings and the case. It is also used to help provide cooling and to prevent the formation of moisture, which can d eteriorate the winding insulation.

There are only 4 possible transformer combinations: Delta to Delta – use: industrial applications

Delta to Wye – use : most common   for step-up   transformer; commercial and industrial

Wye to Delta – use :   most common for step-down   high voltage   Wye to Wye – use : rare, don’t use causes harmonics an d balancing problems.

Characteristics of Distribution Transformer:

1. According to method of cooling

a. Oil-immersed, combination self-cooled and fan -cooled

2. According to insulation between windings a. Windings insulated from each other

b. Autotransformers

3. According to number of phases a. Poly-phase

4. According to method of mounting a. Platform

5. According to purpose

a. Constant-voltage

b. Variable-voltage

6. According to service a. large power

b. Distribution

Bus-bar Arrangement

Bus-bars   are   the   important   components   in   a   substation   .there   are   several   bus-bar   arrangement that   can   be   used   in   substation   .The   choice   of   a   particular   arrangement   depends   upon   various factors such   as   voltage,   position   of   substation, degree   of reliability,   cost   etc.   These   are made   up of   copper   and   aluminum   to   which   the   terminal   of   generators,   transformers,   distribution   lines, loads   etc   is connected.   In an   electrical power distribution system that conduct   electricity within   a switchboard,   distribution   board,   substation,   or   other   electrical   apparatus.   These   bus-bar   are insulated from each other and also from the earth.

The   size   of   the   bulbar   is   important   in   determining   the   maximum   amount   of   current   that   can   be safely   carried.   Bus   bars   can    have   a   cross-sectional   area   o f   as   little   as   10 mm²   but   electrical substations may use metal tubes of 50 mm in diameter (1,963 mm²) or more as bus bars.

The following are the important bus-bar arrangements used in substation.

• Single busbar

• Single busbar system with sectionalisation

• Double/ Duplicate bus-bar arrangement

Duplicate type busbar

This   system   consists   of   two   bus-,a   main   bar-bar   and   a   spare   bus-bar.    Each   bus bar   has   the capacity to   take up the entire substation load .The   incoming and outgoing lines can   be connected

to   either   bus-bar   with   the   help   of   a   bus-bar   coupler   which   consists   of   a   circuit   breaker   and

Isolators.   The   incoming   and   outgoing   lines   remain   connected   to   the   main   bus bar.   However,   in case of   repair of   main bus-bar   or fault   occurring on   it,   the continuity of   supply to the circuit can

be maintained by transferring it to the spare bus-bar.


The   insulator   serves   two   purpose.   They   support   the   conductor   (or   bus   bar   )   and   con fine   the current to the conductor.   The most commonly used material   for the manufactures of insulators is porcelain.   There are   several type of insulator (i.e. pine   type, suspension type etc.) and there used in Sub-Station will depend upon the service requirement.

Earth system :

Why ground?

Poor   grounding   not   only   contributes   to   unnecessary   downtime, but   a   lack   of   good   grounding   is also   dangerous   and   increases   the   risk   of   equipment   failure   .Without   an   effective   grounding system   ,we could be exposed   to   the   risk of   electric shock , not to   mention instrumentation errors ,harmonic   distortion issues, power   factor problems   and a host   of possible   intermittent dilemmas.

If fault currents have no path to the   ground   through   a   properly   designed   and   maintained   grounding   system,   they   will   find unintended   paths   that   could   include   people .The   following   organizations have   recommendations and/or standards for grounding to ensure safety:

• OSHA (Occupational Safety Health Administration)

• NFPA (National Fire Protection Association)

• ANSI/ISA (American National Standards Institute and   Instrument Society of America)

• TIA (Telecommunications Industry Association)

• IEC (International Electro-technical Commission)

• CENELEC (European Committee for Electro-technical Standardization)

• IEEE (Institute of Electrical and Electronics Engineers)

However,   good   grounding   isn’t   only   for   safety;   it   is   also   used   to   prevent   damage   to   industrial plants   and  equipment.   A   good   grounding   system   will   improve   the   reliability   of   equipment   and reduce   the   likelihood of   damage   due to lightning   or   fault   currents .Billions   are lost each   year   in

the workplace due   to electrical   fires. This does not account for   related   litigation costs and loss of personal and corporate productivity.

Why test grounding systems?

Over   time,   corrosive   soils   with   high   moisture   content,   high   salt   content,   and   high   temperatures can   degrade   ground   rods   and   their   connections.   So   although   the   ground   system   when   initially installed,   had   low   earth   ground   resistance   values,   the   resistance   of   the   grounding   system   can increase   if   the   ground   rods are eaten   away.   With frustrating,   intermittent electrical problems, the problem   could   be   related   to   poor   grounding   or   poor   power   quality   .That   is   why   it   is   highly recommended that   all grounds   and   ground   connections are checked   at least   annually   as   a   part of

your   normal   Predictive   Maintenance   plan.   During   these   periodic   checks,   if   an    increase   in resistance   of   more   than   20   %   is   measured,   the   technician   should   investigate   the   source   of   the problem,   and make   the correction to lower the resistance, by   replacing   or   adding   ground rods   to

the ground system.

What is a ground and what does it do?

The NEC,   National   Electrical   Code,   Article   100   defines   a   ground   as:   “a conducting connection, whether intentional   or   accidental   between   an   electrical   circuit   or equipment   and   the   earth,   or   to some   conducting   body   that   serves   in   place   of   the   earth.”   When   talking   about   grounding,   it   is actually two   different subjects:   earth grounding   and   equipment   grounding. Earth   grounding is   an intentional   connection   from   a circuit conductor,   usually   the   neutral, to a   ground electrode   placed in   the   earth.   Equipment   grounding   ensures   that   operating   equipment   within   a   structure   is

properly   grounded.   These   two   grounding   systems   are   required   to   be   kept   separate   except   for   a connection   between    the   two   systems.   This   prevents   differences   in   voltage   potential   from   a

possible   flashover   from   lightning   strikes.   The   purpose   of   a   ground   besides   the   protection   of people,   plants   and   equipment   is   to   provide   a   safe   path   for   the   dissipation   of   fault   currents, lightning strikes, static discharges, EMI and RFI signals and interference.

What is a good ground resistance value?

There   is   a   good   deal   of   confusion   as   to   what   constitutes   a   good   ground   and   what   the   ground resistance value needs to be. Ideally a ground should be of zero ohms resistance .There is not one

Standard ground resistance   threshold that   is recognized   by   all agencies. However,    the NFPA   and IEEE   have   recommended   a   ground   resistance   value   of   5.0 ohms or   less.   The   NEC   has   stated   to “Make sure   that   system   impedance   to   ground   is   less   than   25   ohms   specified   in NEC   250.56.   In facilities   with   sensitive   equipment   it   should   be   5.0   ohms   or   less.”   The   Telecommunications industry   has   often   used   5.0   ohms   or   less   as   their   value   for   grounding   and   bonding   .The   goal   in ground   resistance   is   to   achieve   the   lowest   ground   resistance   value   possible   that   makes   sense economically and physically.

Components of a ground electrode

• Ground conductor

• Connection between the ground   conductor and the ground electrode

• Ground electrode

Locations of resistances

(a)The ground electrode and its connection

The resistance of the ground electrode and   its connection is  generally very   low. Ground rods are generally made of highly conductive/low resistance material such as steel or copper.

(b) The contact resistance of the surrounding earth to the electrode

The National Institute of   Standards   (a   governmental   agency   within   the US   Dept.   of Commerce)has   shown   this   resistance   to   be   almost   negligible   provided   that   the   ground   electrode   is   free   of paint, grease,   etc. and that the ground electrode is in firm contact with the earth.

(c) The resistance of the surrounding body of earthThe ground   electrode is   surrounded by   earth which conceptually   is made   up   of concentric   shells all   having   the   same   thickness.   Those   shells   closest   to   the   ground   electrode   have   the   smallest amount   of area   resulting   in the   greatest   degree of   resistance.   Each subsequent shell   incorporates

a greater area resulting   in   lower resistance.   This   fin ally   reaches   a   point   where   the   additional   shells   offer   little resistance   to   the   ground    surrounding   the   ground    electrode.   So   based   on   this   information,   we should focus on ways to reduce the ground resistance when installing grounding systems.

What affects the grounding resistance?

First, the NEC   code   (1987, 250-83-3) requires   a minimum ground   electrode length of   2.5 meters (8.0 feet)   to be in contact  with soil. But, there are four   variables that   affect the ground resistance of a ground system:

1. Length/depth of the ground electrode

2. Diameter of the ground electrode

3. Number of ground electrodes

4. Ground system design

Length/depth of the ground electrode

One   very   effective way   of   lowering   ground   resistance   is   to   drive   ground   electrodes   deeper.   Soil is not consistent in its resistivity and can be highly unpredictable. It is critical when installing the ground electrode, that it   is   below   the   frost line.   This   is done   so that the resistance to   ground will not   be   greatly   influenced   by   the   freezing   of   the   surrounding   soil.   Generally,   by   doubling   the length   of   the   ground   electrode   you   can   reduce   the resistance  level   by an addition   a   l0   %.   There are   occasions   where   it   is   physically impossible   to   drive   ground   rods   deeper—areas   that   are composed   of   rock,   granite,   etc.   In   these   instances,   alternative   methods   including   grounding cement are viable.

Diameter of the ground electrode

Increasing   the diameter   of   the   ground   electrode   has   very   little   effect   in   lowering   the   resistance. For   ex ample,   you   could   double   the   diameter   of   a   ground   electrode   and your   resistance   would only decrease by 10 %.

Number of ground electrodes

               Figure -: Each ground electrode has its own ‘sphere of influence’.

Another way to lower ground resistance is to use multiple ground electrod es.   In this design, more than one electrode is driven into the ground and   connected in parallel to   lower the resistance. For additional electrodes to be effective, the spacing of additional rods need to be at least equal to the depth   of   the   driven   rod.   Without   proper   spacing   of   the   ground   electro des,   their   spheres   of influence will intersect and   the   resistance will not be   lowered.   To   assist you in installing a   g that will   meet   your   specific   resistance   requirements,   you   can   use   the   table   of   ground   resistances, below.   Remember,   this   is   to   only   be   used   as   a   rule   of   thumb,   because   soil   is   in   layers   and   is rarely homogenous. The resistance values will vary greatly.

Ground system design

Simple grounding systems consist of a single ground electrode driven into the ground. The use of a   single   ground electrode   is the   most common   form   of   grounding   and can   be found   outside your home   or   place   of   business.   Complex  grounding   systems   consist   of   multiple   ground   rods, connected,   mesh   or   grid networks,   ground   plates, and   ground loops.   These systems   are   typically installed at power generating substations,   central   offices, and cell   tower sites. Complex networks dramatically   increase   the   amount   of    contact   with   the   surrounding   earth   and   lower   ground resistances.

Figure: –   Mesh network,   Ground plate.

How do I measure soil resistance?

To   test   soil   resistivity,   connect   the   ground   tester   as   shown   below.   As   you   can   see,   four   earth ground   stakes   are   positioned   in   the   soil   in   a   straight   line,   equidistant   from   one   another.   The distance   between   earth   ground   stakes   should be at   least three   times   greater than   the   stake depth. So   if   the   depth   of   each   ground   stake   is   one   foot   (.30meters),   make   sure   the   distance   between stakes   is greater than three   feet (.91   meters). The Fluke   1625   generates a known current   through the two   outer   ground stakes and   the   drop in voltage   potential is   measured between   the   two inner

Ground   stakes.   Using   Ohm’s   Law   (V=IR),   the   Fluke   tester   automatically   calculates   the   soil resistance.   Because   measurement   results   are   often   distorter   and   invalidated   by   underground pieces of metal, underground aquifers,   etc. additional measurements where the stake’s axis are turned 90   degrees is always recommended. By changing the depth and distance several times, a profile is produced that can determine a   suitable ground   resistance   system.   Soil   resistivity   measurements   are   often   corrupted   by   the   existence   of ground   currents   and   their   harmonics.   To   prevent   this   from   occurring,   the   Fluke   1625   uses   an Automatic   Frequency   Control   (AFC)   System.   This   automatically   selects   the   testing   frequency with the least amount of noise enabling you to get a clear reading.

What are the Methods of Earth Ground Testing?

Fall-of-Potential measurement

The Fall-of-Potential test   method is used   to   measure the   ability   of   an   earth ground   system   or   an individual electrode to dissipate energy from a site.

How does the Fall-of-Potential test work?

First, the earth electrode of interest must be disconnected from its connection to the site. Second,   the tester   is   connected   to   the earth   electrode.   Then,   for   the   3-pole Fall-of-Potential test, two earth   stakes are placed in the soil   in a direct   line—away   from the earth   electrode. Normally,

spacing   of   20   meters   (65   feet)   is   sufficient.   For   more   detail   on   placing   the   stakes,   seethe   next section. A known current is   generated by the Fluk e 1625 between the outer stake (auxiliary earth stake) and the   earth   electrode, while the drop in

Voltage potential is measured between thee earth stake and the earth electrode. Using Ohm’s   Law

(V = IR ),   the   tester   automatically   calculates   the   resistance   of   the   earth   electrode.   Connect   the ground tester   as shown   in   the   picture. Press START and read out   the   RE   (resistance) value.   This is   the   actual   value   of   the   ground   electrode   under   test.   If   this   ground   electrode   is   in   parallel   or series with other ground rods, the RE value is the total value of all resistances.

How do you place the stakes?

To   achieve   the   highest   degree of accuracy when   performing a   3–pole ground resistance   test, it is essential   that   the   probe   is   placed   outside   the   sphere   of   influence   of   the   ground   electrode   under test and the   auxiliary   earth.   If   you   do not   get   outside the sphere of   influence, the   effective areas

of   resistance   will   overlap   and   invalidate   an y  measurements   that   you   are   taking.   The   table   is   a guide for   appropriately   setting the   probe   (inner   stake) and auxiliary   ground (outer   stake).To test the   accuracy   of   the   results   and   to   ensure   that   the   ground   stakes   are   outside   the   spheres   of influence,   reposition   the   inner   stake(probe)   1   meter   (3   feet)   in   either   direction   and   take   a   fresh measurement.   If   there   is   a   significant   change   in   the   reading   (30   %),   you    need   to   increase   the distance   between the ground rod   under test, the inner stake (probe)   and the outer stake (auxiliaryground)   until   the   measured   values   remain   fairly   constant  when  repositioning   the   inner   stake.


11/0.415kV, 7MVA   Sub-station of Instrument is …

a) 7MVA 33/11Kv Main Transformer,4 MVA.3 MVA.2 MVA.&1.5 MVA kVA Transformer 11kV/0.415kV

b) LT Switchgear-5000A 4000A 3000A

c) Dropout Fuse, Rated Voltage (Nominal) -11kV, Rated Current RMS -100A,1Set

d) Lightning Arrester , Rated Voltage (RMS) – 9kV, 1Set

e) 7Mva – 3 Units1500 KVAR-tow units,150KVAR 5-Units Automatic   PFI   Plant.

f) Current Transformer,   Ratio 2500/5A 1000/5A-500/5A,

g) Potential Transformer, Ratio -11//110V , 3pcs.

Setup Information

Factory   requirement   following   as   there   instrument   will   be   ascension.   There   Instrument   of   the figure      same   as   connection   from   the   distribution   line.   I   always   help   them   to connection and working time.

First   time   we are   connected   Isolator.   It   connect   to   main line   include   drop   out   Fuse   15A,   nos.   of

3pcs,   for   3   phase.   Isolator   one   terminals   connect   to   main   distribution   line   (11kv   line)   another terminals connect to current transformer   of primary side. This Isolator attach on the front of poll. After   transformer   connection.   Transformer   connected   is   delta   to   star,   primary   side   is   delta   and secondary   side   is   star.   primary   side   voltage   is   11kV   and   secondary   voltage   is   420V. Secondary side   of   common   terminal   connected   to   grounding   and   also   transformer   body   connected   to grounding.   Primary   side   bushing   number   is   3pcs.   and   secondary   side   bushing   number   is

4pcs.Transfomer   all   of   check,   oil   level,   silica   gel,   temperature,   and   all   nut   bolt.   Transformer secondary side connect to LT switch   gear (Low tension) its medium is LT cable.

Now   Low tension   switch   gear   connection. Low   tension indoor type switchgear   with   hard   drawn copper   bus-bars,   TPN&E   equipped   with   incoming      connected   is   1no.   500A,   36kA,   TP   MCCB with   adjustable   thermal   overload   and   adjustable   magnetic   short-circuit   releases.   3nos.   current transformer,   ratio:500/5A   with   suitable   accuracy   and   burden.   3nos.    Ammeter,   0-500A.

1no.Voltmeter, 0-500V   with selector switch. 2nos. indicating lamp   on/off    and 1 set control fuse.

Out   going   is   1no.   250A,36kA,TP   MCCB   with   adjustable   thermal   overload      and   adjustable magnetic   short   circuit   releases.   3nos.   100A,   16kA, TP   MCCB   with   adjustable thermal   overload and   adjustable   magnetic   short   circuit   releases.   2nos.   160A,   16kA,   TP   MCCB   with   adjustable thermal overload    and   adjustable magnetic short   circuit releases. Low   tension switch   gear   by the distributed   to   factory   machineries.   And   power   factor   improvement   plant   connected   from   low tension switchgear.

Sub-station   last   pert   is   150kVAR    automatic   power   factor   improvement   plant   connected.   It   is flour mounting, 415V, 50HZ, 150kVAR indoor type Automatic   power factor improvement plant,

comprising: 1no. 12.5kVAR bank of TP dry type power capacitor with built-in   discharge   resistor

(Direct). 1no. 12.5kVAR bank of  TP dry  type  power  capacitor   with   built-in  discharge  resistor.

3nos. 25kVAR  bank  of  TP  dry  type  power  capacitor with   built-in  discharge  resistor.   1no.

50kVAR   bank   of   TP   dry   type   power   capacitor   with   built-in   discharge   resistor.1no.   automatic power   factor   correction   relay.   5   nos.   TP   air   contractors   of   adequate   rating.   18   nos.   HRC   fuses with base of adequate rating. 5 nos. indicating lamps and 1 set control fuses. And   other   give  the  connected  on  Distribution   Box,   Switching   bo ard  etc.  There  are  connected from  low  tension   switchgear.  Distribution   Box  to  connect   the  Machineries   line.   And   all Instrument & machineries of   body connected to earth grounding.

Operation of Sub-station:

At   many   places   in   the   line   of   the   power   system,    it   may   be   desirable   and    necessary   to   change some   characteristic   (v oltage,   frequency,   power   factor   etc.)   of   electric   supply.   this   is

accomplished   by   suitable   apparatus   called   sub-station.   The   sub-station   operation   explained   as under:

1)   The   3-phase,   3-wire   11kV   line   is   tapped   and   brought   to   th e   gang   operating   switch installed      near   the   sub-station.   The   G.O.   switch   consists   of   isolators   connected   in    each phase of the 3-phase line.

2)   From   the   G.O.   switch,   the   11kV   line   is brought   to   the indoor   sub-station as   underground cable.   It is   fed to the H.T.   side of   the   transformer (11kV/400V) via the 11kV O.C.B. The transformer steps down the voltage to 400V, 3- phase, 4 wire.

3)   The secondary   of transformer   supplies to the bus-bars   via   the   main O.C.B. From the bus- bars,400V,   3   phase,   4-wire   supply   is   given   to   the   various   consumers   via   400V   O.C.B. The voltage between any phase and neutral it is 230V. The single phase residential load is

connected   between   any   one   phase   and   neutral   whereas   3-phase,   400V   motor   load   is connected   across 3 -phase lines directly.

4)   The   CTs   are   located   at   suitable   place   in   the   sub-station   circuit   and   supply   for   the metering   and indicating instruments and relay circuits.

Maintenance and Trouble shutting

1   Symmetrical Fault   The   symmetrical fault   rarely  The   symmetrical   fault   is   the occurs   in   practice   as  most severe   and imposes more majority   of   the   fault   are   of    heavy   duty   on   the   circuit unsymmetrical nature breaker.

The   reader  to understand the problems   that   short   circuit conditions   present   to   the power system.

2   Single  line to ground Any line with short to the Separate to   the   line from short fault.ground fault.circuit to solve the problem.

Insulation problem.

3   Line to line fault.   One line with another line toSeparate to   the   line from short short.circuit   for   solving the problem.Insulation problem.

4   Double   line   to   ground   Two   line   with   short   to   the     Separate to   the   line from short fault.

Insulation problem.


5   Arc phenomenon   When   a   short-short   circuit Arc   resistance   is   made   to occurs,  a   heavy   current   increase   with   time   so   that flows   the   contacts  of   the current   is   reduced   to   a   value circuit breaker. Insufficient   to   maintain   the arc.

The   ionized   particles   between the   contacts   tend   to   maintain the arc.

6   Transformer open circuit  An open circuit in one phase Open   phase   connect   with   to fault. of   a   3-phase   transformer             circuit may   cause   undesirable heating.

Relay protection is not provided against open circuits

On   the   occurrence   of such  a  because   this   condition   is fault, the transformer   can be  relatively harmless. disconnected   manually from the system.

7   Transformer  overheating Over heating   of  The relay   protection   is   also fault.transformer   is   usually  not   provide against   this caused   by   sustained contingency   and   thermal overloads   or   short-circuit accessories   are   generally   used and very   occasionally by the to   sound   an   alarm   or   control failure   of   the   cooling   the bank of fans.


8   Transformer  Winding   short-circuit   (also  The   transformer   must   be short circuit fault.  called   internal   faults)   on   the     disconnected   quickly from the

transformer   arise   from system   because   a   prolonged deterioration  of   winding  arc   in   the   transformer   may insulation   due   to cause oil fire.

overheating   or   mechanical injury.

9   Lightning for   over   voltage   The   surges   due   to   internal        Surges   due   to   internal   causes fault. causes   hardly   increase   the          are   taken care   of   by   providing

system   voltage   to   twice   the      proper   insulation   to   the normal value.  equipment   in   the   power system.

A   lightning   arrester    is   a protective   device   which conducts   the   high   voltage surges on   the power   system   to the ground.

10   Low voltage   Supply voltage is low. Transformer   tap changing   turn to   move   after   solved   the problem.


The   term   switchgear,   used   in   association   with   the   electric   power   system,   or   grid,   refers   to   the electrical   equipments   like   isolators,   fuses,   circuit   breakers   which   intended   to   connect   and disconnect   power   circuits   are   known   collectively   as   switchgear.   Switchgear   is   used   in   connect with   generation,   transmission,   distribution   and   conversion   of   electric   power   for   controlling, metering   protecting   and   regulating   devices.   A   basic   function   of   switchgear   power   systems   is protection   of   short   circuits   and   overload   fault   currents   while   simultaneously   providing   service continuously   to   unaffected   circuits   while   avoiding   the   creation   of   an   electrical   hazard. Switchgear   power   systems   also   provide   important   isolation   of   various   circuits   from   different

power supplies   for safety   issues. There   are many different   types and   classifications of switchgear power systems to meet a variety of different needs.Switchgear power   systems   can   vary,   depending   on several   factors, such   as power   need, location of   system   and   necessary   security.   Therefore,   there   are   several   different   types   of   switchgear

power   systems   and   each   has   their   own   unique   characteristics   to   meet   the   specific   needs   of   the system and its location.

Switchgear   instruments of Factory

Factory   has   low   voltage   (up   to   380   volt)   and   medium   voltages   (up   to   400V)   switch   gear.   It   is indoor type and switch gear instruments are:

1)   Circuit   breaker   –     Miniature   circuit   breaker,   Vacuum   circuit   breaker,   molded   case   circuit breaker.

2)   Relay   –   Distance   Relay,   Over   current   and   Earth   fault   relay,   Under/Over   voltage   relay,   Trip circuit supervision relay, Differential protection relay, Static relay

3)   Current transformer (C T)

4)   Potential transformer (PT)

5)    Fuse

6)    Lightning   arrestor

7)   Isolator   and Earth switches

8)   Magnetic conductor

Current transformers (CT):

Current   Transformers   are   used   in   current   circuits   in   protection   systems   employing   secondary relays.    This   transformer   is   to   measure   large   currents   and   differs   in   phase   from   it   by   an   angle which   is   approximately   zero   for   an   appropriate   direction   of   the connections.   This   highlights the accuracy   requirement of   the current   transformer but also   important is   the   isolating   function. The

primary which is usually of few turns or even a single turn or thick copper or brass bar is inserted into   the   core   of   the   transformer   is   connected   in   series   with   the   load.    The   secondary   current   is normally   rated   for    5A   or    1A   and   the   number   of   turns   in   the   secondary   will   be   high.   When   the current transformer has   two   secondary   windings then one winding is   connected   to   the protective

relay   system   and   the   other   is   to   indicating   /   metering   circuit.   Current   transformer   windings   are polar in nature.    The current transformers   with   1A   rating   secondary   can   handle   25   times more burden   than   the   current   transformers   of   5A   secondary.    Current   Transformers   of 1ASecondaries   are normally used in the protection of 11 kV – 132 kV.

Transmission lines   where   the   substation apparatus   is   located   at   a   considerable distance from the control   room,   where   the   relays   are   situated.   The   magnitude   of   the   current   which   flows   through the secondary winding of   a   CT   is a function   of the   primary   current,   the   transformation ratio   and

also   the   impedance of   the secondary circuit.   CT’s   normally   operate under       Conditions   close   to short   circuit   conditions.    The   Secondar y   winding   burden   further   depends   upon   the   method   of connection   of   the   C T   secondary,   the   relay   windings   and   the   kind   of   short   circuit   experienced. CT’s used   for extra   high voltage net work protection must be   capable   of accurately   transmitting currents   both   during   steady   state   process   and   under   transient   conditions   in   order   to   permit operation   of   the   protective   devices   correctly.   The   reasons   for   choosing   proper   CT’s   for   extra high voltage net work protection are;

1. The time constants of DC components in the short circuit currents of EHV net works are large.

2.   The   ratio of   the short   circuit   current   to   the   rated   current   is   very   high, due to increased energy concentration.

3.   High   Speed   relaying   is   essential   to   protect   electrical   equipment   during   fault   and   to   increase system stability

The accuracy of a CT is directly   related to   a number of factors including:


Burden class/saturation class

Rating factor


 External electromagnetic fields

Temperature and

Physical configuration.

The selected tap, for multi-ratio CT’s

 Ratio 5000/5A

Potential Transformers (PT):

Instrument   Transformers   are   of   means   of   extending   the   ran ge   of   AC   in str umen ts   like ammeters,    voltmeters,    V.A.R.   meters,   Walt-meters.   They   are   two   types   of   potential transformers.   The primary of   the   potential transformers is   connected   across the transmission line whose voltage   may   range   from 2.4   kV to   220 kV.   The   secondary   voltage is   standardized at 110 kV.   The load connected to the secondary is   referred to as   burden. The requirements of   the good potential transformers   are:

Accurate turns  ratio ,    n    =    V p/ Vs

The   difficulty   in   maintaining   the   accurate   turn’s   ratio   is   due   to   resistance   and   reactance   of   the windings and the value of the exciting current of the transformer.

Small   leakage   reactance.   The   leakage   reactance   is   due   to   the   leakage   of the   magnetic   fluxes   of the primary   and   secondary voltages.   They can   be   minimized   by   keeping the   primary, secondary windings   as   close   as   possible   subject   to   insulation   problem   as   the   primary   is   at   high   voltage.

Small   magnetic   current.   This   can   be   achieved   by   making   the   reluctance   of   the   core   as   small   as possible   and   flux   density   in   the   core   is   also   lowland   it   is   very   less   than   1   wb   /   m      Minimum Voltage   Drop:    The   resistance   of   the   windings   is   made   as   small   as   possible   The   Primary   as   it carries   high   voltage   should   be   heavily   insulated.    Hence   it   is   immersed   in   oil   and   the   terminals are brought out to   porcelain   bushing.   Now-a-days   synthetic rubber insulation like   styrene   is used avoiding   oil   and   porcelain.      When   the   load   or   burden   on   the   secondary   is   increased,   the secondary   current   increases   with   corresponding   increase   in   primary   current   so   that transformation ratio remains the same

Specification of Potential Transformer

• Manufacturer                                : ABB

• Device maximum operating voltage   : 35kV

• Rated frequency                                : 50-60Hz

• Rated voltage                                     : 11 kV

• Rated output                                       : 440V

Lightning Arrestor

A lightning arrester   is a   device used   on   electrical power   systems to   protect   the   insulation   on the system   from the damaging effect   of lightning. Metal   oxide   varistors (MOVs) have been   used for power system protection since the mid 1970s. The typical lightning arrester   also known as   surge

arrester has a   high   voltage   terminal   and   a   ground terminal.   When a lightning   surge   or switching surge travels down the   power system to the arrester, the current from the surge is diverted around

the protected insulation in most cases to earth.

Specification of Lightning Arrestor

• Manufacturer                                : ABB

• Rated voltage                                : 11kV

• Rated current                                 : 50-60Hz

• Rated discharging current              : 10kA

• Continuous operating voltage        : 100.8kV

• Residual voltage under thunder In : 10kA         615kV peak

• Standard discharging current                  : 1.0kAPeak

Isolators and earth switches :

Isolator   is   a   no-load   switch   designed   as   a   knife   switch   to   operate   under   no-load   conditions therefore   the   isolator   o pens   only   after   the   opening   after   the   circuit   breaker.   While   closing, isolator   closes   first   and   then   circuit   breaker.    Isolator   is   also   called   as   disconnecting   switch   or simply disconnected.    It is interlock with circuit breaker such that wrong operation is avoided. Its

main   purpose   is   to   isolate   one   portion   o f   the   circuit   from   the   other   and   is   not   intended   to   be

opened   while   current   is   flowing   in   the   line.   Such   switches   are   generally   used   on   both   sides   of circuit breakers in order that repairs and replacement of circuit breakers can be made without any

danger. During   the opening operation the   conducting   rods   swing apart   and   isolation   is   obtained.

The   simultaneous   operation   of   three   poles   is   obtained   by   mechanical   interlocking   of   the   three poles.   Further,   for   all   the   three   poles,   there   is   a   common   operating   mechanism.   The   operating mechanism is manual plus one of the following:

° Electrical motor mechanism

° Pneumatic  Mechanism.

They   should   never   be   opened   until   the   circuit   breaker   in   the   same   circuit   has   been   opened   and should   always   be   closed   before   the   circuit   breaker   is   closed.   .   MPS   has   3   pole   isolators   have three identical   poles. Each pole consists   of three insulator posts mounted on a fabricated support.

The   conducting   parts   are   supported   on   the   insulator   posts.   The   conducting   parts   consist   of conducting copper or aluminum rod, fixed and moving contacts. Isolators installed   in the outdoor

yard   can   be   operated   controlled   manually   or   electrically   on   electrical   mode   both   local   /   remote operations is   possible.    All   circuit   breakers can be operated /   controlled in   electrical   mode   either

local   /   remote   position.   The   remote   control   /   monitoring   of   all   isolators   and   circuit   breakers   is  done with the help of a set of control and metering panels Earth   Switch   is   connected   between   the   line   conductor   and   earth.    Normally   it   is   open   and   it   is closed   to   discharge   the   voltage   trapped   on   the   isolated   or disconnected   line.    When   the   line   is disconnected   from   the   supply   end,   there   is   some   voltage   on   the   line   to   which   the   capacitance between the line   and earth is charged.       This   voltage   is   significant   in    HV   systems .      Before commen cement   o f   maintenance   work   it is necessary that   these   voltages   are discharged to earth

by   closing   the   earth   switch.    Normally   the   earth   switches   are   mounted   on   the   frame   of   the isolator

Sequence of Operation while Opening/closing a circuit:

Opening Sequence:

1. Open Circuit Breaker

2. Open Isolator

3. Close Earth Switch

Closing Sequence:

1. Open Earth Switch

2. Close Isolator

3. Close Circuit Breaker

Circuit Breaker

A   circuit   breaker   is   an   automatically-operated   electrical switch   designed   to   protect   an   electrical circuit   from   damage   caused   by   overload   or   short   circuit.   Its   basic   function   is   to   detect   a   fault condition and these by interrupting continuity, to immediately discontinue electrical flow.

Principle of Operation

All   circuit breakers have   common features   in   their   operation,   although details vary substantially depending on   the   voltage class, current rating   and type of   the   circuit   breaker. The circuit breaker

must   detect   a   fault   condition   in   low-voltage   circuit   breakers   this   is   usually   done   within   the breaker   enclosure. Circuit breakers   for large   currents   or high voltages   are   usually   arranged   with pilot   devices   to   sense   a   fault   current   and   to   operate   the   trip   opening   mechanism.   The   trip

solenoid   that   releases   the   latch   is   usually   energized   b y   a   separate   battery,   although   some   high-

voltage   circuit   breakers   are   self-contained   with   current   transformers,   protection   relays   and   an internal control power source.

Once   a   fault   is   detected,   contacts   within   the   circuit   breaker   must   op en   to   interrupt   the   circuit. Some mechanically-stored energy (using   something   such as springs   or compressed air) contained

within the breaker is   used to separate   the   contacts,   although   some   of the energy required   may   be

obtained   from   the   fault   current   itself.   The   circuit   breaker   contacts   must   carry   the   load   current

without   excessive   heating,   and   must   also   withstand   the   heat   of    the   arc   produced   when interrupting   the   circuit.   Contacts   are   made   of   copper   or   copper   alloys,   silver   alloys   and    other materials.   Service   life   of   the   contacts   is   limited   b y   the   erosion   due   to   interrupting   the   arc. Miniature   circuit   breaker s   are   usually   discarded   when   the   contacts   are   worn,   but   power   circuit breakers and high-voltage circuit breakers have   replaceable contacts.

When   a   current   is   interrupted,   an   arc   is   generated   –   this   arc   must   be   contained,   cooled,   and extinguished   in   a   controlled   way,   so   that   the   gap   between   the   contacts   can   again   withstand   the voltage   in   the   circuit.   Different   circuit   breakers   use   vacuum,   air,   insulating   gas,   or   oil   as   the medium in which the arc forms.

Classification of   Circuit Breaker

According to the voltage level circuit breaker are classified into three categories, such as

1.   Low Voltage Circuit Breaker( Up to 619 volt)

2.   Medium Voltage Circuit Breaker(Up to 11kV)

3.   High Voltage Circuit Breaker(Up to 145kV )

Low Voltage Circuit Breaker

1.  Molded   Case   Circuit   Breaker   (MCCB):       Molded   case   circuit   breaker   operation   as   like   as thermal   or   thermal-magnetic   operation   and   rated   current   start   from100A.   Trip   current   may   be adjustable   in   larger   ratings.   The   molded   case   circuit   breaker (MCCB)   co mprises   the   following features:

• A contact system with arc-quenching   and current-limiting means

• A mechanism to open and close the contacts

• Auxiliaries   which   provide   additional   means   of   protection   and   indication   of   the   switch positions

Case Circuit Breaker

Figure  Molded Case Circuit Breaker

The   MCCB   may   be   used   as   an   incoming   device,   but   it   is   more   generally   used   as   an   outgoing device   on the load   side of   a   switchboard.   It   is normally   mounted into   a low-voltage switchboard

or   a   purpose-design ed   panel   board.   In   addition   to   the   three   features   listed   at   the   start   of   this section, it also includes:

• An   electronic   or   thermal/electromagnetic   trip   sensing   system   to   operate   through   the tripping mechanism and open the circuit breaker under overload or fault conditions

• All parts housed within a plastic molded housing made in two halves

• Current ratings usually from 10A to 1600A.

Miniature Circuit Breaker (MCB):        Miniature   circuit breakers   rated current not   more   than 100A. Trip characteristics   normally   not   adjustable.   The   miniature circuit   breaker   (MCB)   has   a   contact system and means   of arc   quenching, a mechanism and tripping   and protection system to open the

circuit   breaker   under   fault   conditions..   Early   devices   were   generally   of   the   ‘zero-cutting’   type, and   during a short circuit the current   had to pass through   a zero before the arc was extinguished;

this   provided   a   short-circuit   breaking   capacity   of   about   3kA.   Most   of   these   early   MCBs   were

housed   in   Bakelite   moldings.   The   modern   MC B   is   a   much   smaller   and   more   sophisticated device.   All   the   recent   developments   associated   with   molded   case   circuit   breakers   have   been incorporated   into MCBs   to   improve their   performance,   and   with breaking   capacities   of 10   kA   to

16   kA   now   available,   MCBs are used   in   all   areas   of   commerce and   industry   as   a   reliable means of protection. Most MCBs are of single-pole construction for use in single-phase circuits.

Miniature circuit Breaker

Figure – Miniature circuit Breaker

Medium Voltage Circuit Breakers

Medium-voltage   circuit   breakers   rated   between   619   Voltage   and   11   kV   assemble   into   metal- enclosed switchgear   line   ups   for indoor   use in MPS substation. Medium voltage circuit breakers

are   also   operated   by   current   sensing   protective   relays   operated   through   current   transformers. Medium-voltage   circuit   breakers   nearly   always   use   separate   current   sensors   and   protective relays, instead of   relying on built-in thermal or magnetic over current sensors.

Vacuum   circuit   breaker:        Vacuum   circuit   breaker   with   rated   current   up   to   3000   A,   these breakers   interrupts   the   current   by   creating   and   extinguishing   the   arc   in   a   vacuum   container. These are   generally   applied for voltages   up to about 35,000   V   but PS use vacuum circuit breaker

for   11KV   which   corresponds   roughly   to   the   medium-voltage   range   of   power   systems.   Vacuum circuit   breakers   tend   to   have   longer   life   expectancies   between   overhaul   than   do   air   circuit breakers.   Vacuum   circuit   breakers   tend   to   have   longer   life   expectancies   between   overhaul   th an do air circuit breakers.

In   a   vacuum   circuit   breaker,   two   electrical   contacts   are   enclosed   in   a   vacuum.   One   of   the contacts   is   fixed,   and   one   of   the   contacts   is   movable.   When   the   circuit   breaker   detects   a dangerous   situation,   the   movable   contact   pulls   away   from   the   fixed   contact,   interrupting   the current.   Because   the   contacts   are   in   a   vacuum,   arcing   between   the   contacts   is   suppressed,

ensuring   that   the   circuit   remains   open.   As   long   as   the   circuit   is   open,   it   will   not   be   energized.

Vacuum recluses will automatically reset   when   conditions   are   safe   again, closing the circuit   and

allowing   electricity   to   flow   through   it.   Re-closers   can    usually   go    through   several   cycles   before they will need to be manually reset

Vacuum   interrupters,   mounted   vertically   within   the   circuit   breaker   frame,   perform   the   circuit breaker   interruption.   Consisting   of   a   pair   of   butt   contacts,   one   movable   and   one   fixed, interrupters   require   only   a   short   contact   gap   for   circuit   interruption.   The   resulting   high-speed operation   allows   the entire operating sequence,   from fault   to   clear,   to   be   consistently   performed in three cycles or less.

The   primary   connection   to   the   associated   switchgear   is   through   the   six   primary   disconnects

mounted horizontally   at   the   rear of   the   circuit   breaker. Do not   subject the primary disconnects   to rough   treatment.   The operating   mechanism   is   of   the stored   energy   type.   It   uses charged   springs to   perform   breaker   opening   and   closing   functions.   The   operating   mechanism   contains   all necessary controls and   interlocks. It is mounted   at the front   of   the   circuit breaker for   easy access

during inspection and maintenance.

Specification of   Vacuum circuit breaker:

• Rated frequency-50 -60Hz

• Rated making Current-10 Peak kA

• Rated Voltage-11kV

• Supply Voltage Closing-220 V/DC

• Rated Current-1250 A

• Supply Voltage Tripping-220 V/DC

• Insulation Lev el-IMP 75 kVP

• Rated Short Time Current-40 kA (3 SEC)

High-voltage circuit breakers

Electrical   power   transmission   networks   are   protected   and   controlled   b y   high-voltage   breakers.

The   definition   of   high   voltage   varies   but   in   power   transmission   work   is   usually   thought   to   be

72.5 kV or higher. In MPS used SF6 circuit breaker for high voltage in sub station .High-voltage

breakers   are   always   solenoid-operated,   with   current   sensing   protective   relays   operated   through current   transformers.   In   substations   the   protective   relay   scheme   can   be   complex,   protecting equipment and busses from various types of overload or ground/earth fault.


A fuse is a   short piece   of wire or   thin strip which melts when excessive current flows             through   it for sufficient   time.   It   is   inserted   in   series   with   the   circuit   to   be   protected.   Under   normal   operating conditions   the   fuse   element   it   at   a   temperature   below   its   melting   point.   Therefore,   it   carries   the normal   load   current   without   overheating.   However   when   a   short   circuit   or   overload   occurs,   the current   through the fuse   element   increases   beyond its rated capacity. This raises the temperature and the   fuse   element   melts   (or   blows   out),   disconnecting   the   circuit   protected   by Init. electronics   and electrical   engineering   a   fuse   (short   for   fusible   link)   is   a   type   of   sacrificial   over   current   protection device. Its essential component is a metal wire or strip that melts when too much current flows, which

interrupts   the   circuit   in   which   it   is   connected.   Short   circuit,   overload   or   device   failure   is   often   the reason for excessive current.

Fuse Ratings:

Ampere Rating

Each   fuse   has   a   specific   ampere   rating,   which   is   its   continuous   current-carrying   capability. There are different types of fuse used in MPS, rating start from 2A.

Voltage Rating The voltage rating   of a f use   must   be   at least   equal   to   the   circuit voltage.   The voltage rating   of   a fuse can   be higher   than   the circuit voltage, but never lower. A 500   volt fuse,   for example, could be used in a 450 volt circuit, but a 350 volt fuse could not be used in a 500 volt circuit.

Magnetic Contactor

A magnetic contactor is a relay-controlled switch   used to turn a   power control circuit on and off.It   is   electrically   controlled and   uses   less   power   than   other   circuits.   A   magnetic   contactor   comes in different forms and capacities.Magnetic   contactors   are   a   form   of   electrical   relay   found   on   most   electrically   powered   motors. They   act   as   a   go-between   for   direct   power   sources,   and   high-load   electrical   motors   in   order   to homogenize   or   balance   out   changes   in   electrical   frequency   which   may   come   from   a   power supply as well as to act as a safeguard


A magnetic contactor has three parts: power contacts,   contact   springs and auxiliary   contacts. The power contact creates, carries and breaks the current in   a magnetic contactor. The   contact springs create a sufficient amount   of   pressure   on the   contacts. Auxiliary contacts perform   signaling and interlocking   maneuvers.   Contactors   vary   in   size   and   capacity.   In   heavy   duty   magnetic Conductors,   blowout   coils   perform   magnetic   blowouts   so   the   current   can   go   further   with   more power.   Economizer   circuits   decrease   the   power   needed   to   keep   the   contactor   closed;   these   are usually found in direct-circuit contactor   coils working to keep the contactor cooler..


A   basic   magnetic   conductor   has   a   coil   input   that   is   driven   by   either a   DC   or   AC   supply,   and   it can   be   energized   at   the   same   voltage   as   the   motor.   It   can   also   be   controlled   separately   using programmable   controllers   and   low   voltage   pilot   devices.   Most   contactors   handle   lighting,

Heating, electric motors and capacitor banks

Function of Magnetic conductor

Contactors   are   usually fitted on   open   contacts,   an d are   designed   to   suppress   and   control   electric arcs   which   are   produced   by   interrupting   heavy   motor   currents.   They   work   on   the   principle   of electromagnetism   and   the electricity   runs through   the   coil   from   the   core   of   the contactor.   While the core   is   moving,   a   force   is   developed   that   allows   the electromagnet   to   carry   charge   and   hold the   contacts   together.   Once   the   contactor   coil   is   de-energized,   the   spring   of   the   electromagnet returns to its original position.

Specification of Magnetic conductor:

Manufacturer   : Siemens

Model                     : LC1-D1210M7

Origin      : German

Coil Voltage   : 220V AC Voltage   : 415V

Frequency   : 50/60 Hz


A   changeover   switch   for   a   tap   changer   including   a   pair   of   load   switches.   A   diverter   switch allows   load   to   be   diverted   along   a   second   path   when   its   associated   main   switch   is   opened   or closed. An   auxiliary circuit has an auxiliary switch and   a varistor connected in parallel across the

secondary   of   a   transformer.   When   the   auxiliary   switch   is   opened   the   varistor   impedance   is reflected   onto   the   primary   of   the   transformer   which   causes   the   current   in   the   main   switch   to divert   through   the   diverter   switch   so   that   the   main   switch   can   be   opened   or   closed   with substantially no load on it.

Manual Change over Switch

The Manual   change over   switch is wired into your Electrical Distribution   Board in your home or office   allowing   it   to   power   particular   appliances   in   your   home   or   office   by   providing   power   to specific circuits.

The manual change over switch can be used with the remote start button. The   Generator   does   however   need   time   to   get   up   to   speed   before   the   Manual   Change   Over Switch   can   be placed   on “Generator.” The recommended   time for   this is   5   Seconds. Hence when used in conjunction with   a remote start button, the generator should   be started   whilst   the   Manual Change over Switch is in the “Off” position. Once started and run   for the   recommended time the

switch   can   be moved to “Generator” providing power   to   the   relative circuits which   the   generator has been wired up to provide power to.The   Following   are   the   respective   Model   numbers   associated   with   the   Manual   change   over switches   and   their   capability   of single   or three   phase   power.   The   key   on   the   generator   has   to   be in   the ON position   for   the   manual   change   over   switch   to   work. The   manual   change   over   switch does not charge   the   battery so   should the key   be   left in the on   position   the   battery   will   go flat,   if

the generator is not used on a regular basis.

Protective Relaying and Protection

Protective   relays   are   used   to   detect   defective   lines   or   apparatus   and   to   initiate   the   operation   of circuit   interrupting   devices   to   isolate   the   defective   equipment.   Relays   are   also   used   to   detect abnormal   or   undesirable   operating   conditions   other   than   those   caused   by   defective   equipment and either operates   an alarm or initiate operation of circuit- interrupting dev ices A protection relay is   a   device that senses any change in the signal which it   is   receiving,   usually from a current and/or voltage   source.   If the magnitude   of   the incoming   signal   is outside   a   preset   range,   the relay will operate, generally to close or open electrical contacts to initiate some further operation,   for example the tripping of a circuit breaker.

Characteristic of relay:

Protection relays can be classified in accordance with the function which they carry out, their construction, the incoming signal and the type of functioning.

General function:

• Protection.

• Monitoring.

• Control .


• Electromagnetic.

• Solid state.

• Microprocessor.

• Computerized.

• Incoming signal:

• Current.

• Voltage.

• Frequency.

Type of protection

• Over current.

• Directional   over   current.

• Distance.

• Over voltage.

• Differential.

• Reverse power.

over current and over voltage relay:

The   Over   current   and   over   voltage   relay   responds   to   a   magnitude   of   over   current   and   over voltage         above a   specified   value.   There   are   four   basic   types   of   construction:   They   are   plunger, rotating   disc,   static,   an d   microprocessor   type.   In   the   plunger   type,   a   plunger   is   moved   by magnetic   attraction   when   the   cur rent   exceeds   a   specified   value.   In   the   rotating   induction-disc

type, which   is a motor, the disc rotates   by electromagnetic induction when   the   current exceeds   a specified   value. Static types convert   the cur rent   to   a   proportional   D.C   mill   volt signal and apply it to a   level   detector   with   voltage or   contact   output. Such   relays   can   be   designed   to have   various current-versus-time   operating   characteristics.   In   a   special   type   o f   rotating   induction-disc   relay, called the voltage   restrained over current relay.

The   magnitude   of   voltage   restrains   the   operation   of   the   disc   until   the   magnitude   of   the   voltage drops   below   a   threshold   value.   Static   over   current   relays   are   equipped   with   multiple   curve characteristics   and   can   duplicate   almost   any   shape   of   electromechanical   relay   curve.

Microprocessor   relays   convert   the   current   and   voltage   to   a   digital   signal.    The   digital   signal   can then   be   compared   to   the   setting   values   input   into   the   relay.   With   the   microprocessor   relay, various curves or multiple time-delay settings can be input to set the relay operation. Some relays

allow   the   user   to   define   the   curve   with   points   or   calculations   to   determine   the   output characteristics.

 Distance Relay

The distance   relay   responds   to   a   combination   of   both   voltage   and   current.   The voltage restrains operation,   and   the   fault   current   causes   operation   that   has   the   overall   effect   of   measuring impedance.   The   relay   operates   instantaneously   (within   a   few   cycles)   on   a   60-cycle   basis   for values   of   impedance   below   the   set   value.   When   time   delay   is   required,   the   relays   energizes   a

separate   time-delay   relay   or   function   with   the   contacts   or   output   of   this   time-delay   relay   or function performing the   desired output functions. The   relay   operates   on   the   magnitude   of   impedance   measured   by   the   combination   of   restraint voltage and the   operating current passing through it according to   the settings applied to the relay.

When   the   impedance   is   such   that   the   impedance   point   is   within   the   impedance   characteristic circle,   the   relay   will   trip.   The   relay   is   inherently   directional.   The   line   impedance   typically corresponds   to   the   diameter   of   the   circle   with   the   reach   of   the   relay   being   the   diameter   of   the circle.

Differential Relay

The differential   relay is   a   current-operated   relay   that responds   to   the   difference   between   two   or more device   currents   above   a set value. The relay   works on   the   basis of   the   differential principle

That   what   goes   into   the   device   has   to   come   out   .If   the   current   does   not   add   to   zero,   the   error current   flows   to   cause   the   relay   to   operate   and   trip   the   circuit.   The   differential   relay   is   used   to provide   internal   fault   protection   to   equipment   such   as   transformers,   generators,   and   buses.

Relays are designed   to   permit   differences   in   the   input currents   as a result   of current   transformer mismatch and applications where   the input currents come from different   system voltages, such as transformers.   A current differential relay provides restraint coils   on the incoming current circuits.

The   restraint   coils   in   combination   with   the   operating   coil   provide   an   operation   curve,   above

which   the   relay   will   operate.   Differential   relays   are   often   used   with   a   lockout   relay   to   trip   all power   sources   to   the   device   and   prevent   the   device   from   being   automatically   or   remotely   re- energized.   These   relays   are   very   sensitive.   The   operation   of   the   device   usually   means   major problems with the protected equipment and the likely failure in re-energizing the equipment

Directional Over current Relay

A   directional   over   current   relay   operates   only   for   excessive   current   flow   in   a   given   direction. Directional   over   current   relays   are   available   in   electromechanical,   static,   and   microprocessor constructions. An electromechanical overcorrect relay is made directional b y adding a directional

unit   that   prevents   the   over   current   relay   from   operating   until   the   directional   unit   has   operated. The directional unit   responds to the   product   of   the   magnitude   of current,   voltage,   and   the   phase angle   between   them   or   to   the   product   of   two   currents   and   the   phase   angle   between   them.   The value of this product necessary   to provide operation   of the   directional unit is small,   so that   it will not   limit   the   sensitivity of the relay (such   as an   over current relay   that it controls). In most   cases, the directional   element   is mounted   inside the same case   as the relay   it controls. For   example,   an

over   current   relay and a   directional   element   are   mounted in the   same   case,   and   the   combination is   called   a   directional over current   relay.   Microprocessor   relays often   provide a choice as   to the polarizing method   that   can   be   used   in   providing   the direction   of fault,   such   as applying   residual current or voltage or negative sequence current or voltage polarizing functions to the relay.

Distribution board:

A distribution board      (or   panel) is a component of an        electricity   supply system which divides an electrical power   feed   into   subsidiary      circuits, while providing a   protective        fuse or   circuit breaker for   each   circuit,   in   a   common enclosure  . Normally, a main switch   , and   in   recent   boards, one   or   more  Residual-current   devices      (RCD)   or    Residual   Current   Breakers   with   Overecurrent protection   (RCBO), will also be

Maintenance Instruments of List

1 . Multi-meter (AVO)

2. Multi Screw Driver set

3. Pliers

4. Noose Pliers

5. Cutting Pliers

6. Hammer

7. Tester

8. Series Lamp

9. Wire Striper

10. Spanner set

11. Adjustable wrench

12. Clip-on meter

13. Soldering Iron

14. Griper Pliers

15. Punching etc.


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