function of protective relying

 function of protective relying

the various functions of protective relaying are : 

1- the prompt removal of the component which is behaving abnormally by closing the trip circuit of circuit breaker or to sound an alarm

2- to disconnect the abnormally operating part so as to avoid the damage or interference within effective operation of the rest of system

3- to prevent the subsequent faults by disconnecting the abnormally operating part

4- to disconnet the faulty part as quickly as possible so as to minimize the damage to the faulty part itself. for example, there is a winding fault in a machine and if it presists for a long time then there is a possibility of the damage of the entire winding. as against this, if it is disconnected quickly then only few coils may get damage instead of entire winding

5- to restrict the spreading of the effect of fault causing least interference to the rest of the healthy system. thus by disconnecting the faulty part, the fault effects get localised

6- to improve the system performance, system reliability, system stability and service continuity.

the fault can not be completely avoided but can be minimized. thus thr protective relying plays an important role in sensing the faults, minimizing the effects of  faults and minimizing the damage due to the faults

high voltage bushing

 Bushing is defined as a structure carrying one or several conductors through a partition such as a wall or tank etc. and insulating it or them therefrom

 A bushing has to provide : 

- Electrical insulation to the conductor for the working voltage and for various over voltages, which occur in service.

 - Mechanical support against various Mechanical forces.

TYPES OF BUSHINGS

1.1 Application

i) Alternator bushing
ii) Transformer bushing
iii) Bushings for switch gear
iv) Wall bushing or roof bushing
v) Cable end and joint box bushing

1.2 Application:
i) Indoor type
ii) Outdoor type

2.1 Type of Bushing:
n i) Stem type
n ii) Pull-through type

2.2 Type of Bushings:
n i) Condenser core type
n ii) Non-condenser core type
(Hollow Porcelain type)
n iii) Gas filled bushing
n iv) Solid Bushing

Solid Ceramic Bushing

 Epoxy Bushing

3. Type of insulation used in manufacturing Condenser Bushing :

i) Synthetic Resin Bonded Paper (SRBP)
• Oil filled
• Compound filled

ii) Oil Impregnated Paper (OIP)
• iii) Resin impregnated Paper (RIP)

Fuses and Circuit Breakers

 Fuses and Circuit Breakers ;

Fuses and circuit breakers are used to protect equipment or wiring against excessive current. For example, in your home, if you connect too many appliances to an outlet, the fuse or circuit breaker in your electrical panel “blows.” This opens the circuit to protect against overloading and possible fire. Fuses and circuit breakers may also be installed in equipment such as your automobile to protect against internal faults. Figure 2–29 shows a variety of fuses and breakers.

Fuses use a metallic element that melts when current exceeds a preset value. Thus, if a fuse is rated at 3 A, it will “blow” if more than 3 amps passes through it. Fuses are made as fast-blow and slow-blow types. Fastblow fuses are very fast; typically, they blow in a fraction of a second. Slowblow fuses, on the other hand, react more slowly so that they do not blow on small, momentary overloads.

Circuit breakers work on a different principle. When the current exceeds the rated value of a breaker, the magnetic field produced by the excessive current operates a mechanism that trips open a switch. After the fault or overload condition has been cleared, the breaker can be reset and used again. Since they are mechanical devices, their operation is slower than that of a fuse; thus, they do not “pop” on momentary overloads as, for example, when a motor is started.

Electric car

An electric car is powered by an electric motor instead of a gasoline engine. The electric motor gets energy from a controller, which regulates the amount of power—based on the driver’s use of an accelerator pedal. The electric car (also known as electric vehicle or EV) uses energy stored in its rechargeable batteries, which are recharged by common household electricity.

Electric cars produce no tailpipe emissions, reduce our dependency on oil, and are cheaper to operate. Of course, the process of producing the electricity moves the emissions further upstream to the utility company’s smokestack but even dirty electricity used in electric cars usually reduces our collective carbon footprint.

eddy current

 eddy current

When current is induced in a conductor such as the square piece of metal , the induced current  often flows in small circles that are strongest at the surface and penetrate a short distance into the material. These current flow patterns are thought to resemble eddies in a stream, which are the tornado looking swirls of the water that we sometimes see. Because of this presumed resemblance, the electrical currents were named eddy currents. 

Eddy currents are often generated in transformers and lead to power losses. To combat this, thin, laminated strips of metal are used in the construction of power transformers, rather than making the transformer out of one solid piece of metal. The thin strips are separated by insulating glue, which confines the eddy currents to the strips. This reduces the eddy currents, thus reducing the power loss.

dc machine

 dc machine

D.C. machines are the electro mechanical energy converters which work from a d.c. source and generate mechanical power or convert mechanical power into a d.c. power. These machines can be broadly classified into two types, on the basis of their magnetic structure. They are,

1. Homopolar machines
2. Heteropolar machines.

PDF

Circuit breaker Data Sheets

data sheets of circuit breaker :

- manufacturing

- type of circuit breaker

- rated voltage

- rated lighting impulsed withstand voltage

- rated normal current

- rated duration shot circuit

- rated short circuit breaking current

- press.for interruption ( at 20 )

- mass of complete circuit breaker

- mass of sf6 gas

- rated supply voltage

- year of manufacture

On load tap changers data

 data of On load tap changers : 

- Referred standard and relevant specification

- Insulation level : The following values occurring on all tapping positions of the transformer should be checked against the applicable technical guide.

1- Power-frequency voltage appearing in the tapchanger during test or in service.

2- Impulse voltage appearing on the tap-changer during test on the transformer or in service

- Highest phase service voltage across the regulating winding

Example

115 kV ± 16 % regulating range Y connected, gives the parameter in question as

115 · 0.16 / √3= 10.6 kV

This value has to be compared with highest permissible phase service voltage across the regulating winding
detailed in the applicable technical guide

parameter may in some rare cases be a limiting factor.

- Rated through current

The rated through current of the tap-changer should be not less than that resulting from the highest value of
tapping current of the tapped winding of the transformer.

- Occasional overloading

- Short-circuit current strength
The tap-changer short-circuit current is given in the applicable technical guide.

- Phase step voltage

If the step voltage varies over the range, full details together with associated currents is needed to calculate
the maximum breaking capacity. Limit values are given in the applicable technical guide.

- Tapping range and tapping arrangement

The number of tapping positions has to be considered when selecting type of tap-changer

- Recovery voltage

If the service voltage and the winding capacitance exceed certain limit values stated in the technical guide, tie-in resistors have to be used in order to avoid difficulties with regard to dielectric stresses within the change-over selector in the tap-changer.

- Contact life

The predicted contact life is a function of the rated through current and has to be compared with the requirements.

- Temperature conditions

The temperature of the oil or ambient air temperature surrounding the tap-changer can be in the range between –25 °C and +105 °C during normal service without any special precautions. The range can be extended to –40 °C provided the oil viscosity is between 200 to 800 mm2/s (cst).

High Voltage Disconnector datasheet

High Voltage Disconnector datasheet ; 

- type

- year of manufacture

- rated current

- short circuit current

Fault and abnormal condition

   Fault and abnormal condition

 During a fault, the fault impedance is low and accordingly, the fault currents are relatively high. Since the fault currents being excessive, they damage the faulty equipment and the supply installation. During the faults, the power flow is diverted towards the fault; and the supply to the neighboring zones is affected

Faults can be classified as :

- single line to ground fault

- lint to line fault

- double line to ground fault

- simulation fault

- three phase fault

-                                                                               - open circuit, etc.

The other abnormal condition

-                                                                               - Voltage and current unbalance

- Undr frequency

- Over voltages

 Temperature rise-

-  Reverse of power

 Power swing-

- Instability, etc

Fault clearing process

1- the protective relay are connected in the secondary circuit of current and voltage transformer.

2- the relays the abnormal condition and close the trip circuit of the associated circuit breaker.

3- the circuit breaker opens its contact.

4- arc is drawn between the contacts as they separate.

5- the arc is extinguished by suitable medium and technique.

6- after final arc extinction, ahigh voltage wave appears across the circuit breaker tending to re-establish the arc.

7- the transient voltage wave is called " transient recovery voltage" ( TRV ).

off load tap changer data

off load tap changer data :

- Model CAPT series

- number of phases : ( single-phase , Two-phase and Three-phase )

- number of rod : ( simple , double , triple and other )

- Type of connection : ( Bridging , Linear with a common for each phase , Star (WYE) , Series/Parallel , Star/Delta and combination )

- number position :

- Voltage (kV)

- Ampere (A) : 10A        30A           60A          120A          150A

- Drive handle : Polyamide (nylon) or Aluminium

Transformer data sheets

what are transformer data sheets :

- transformer no 

- manufacturer

- serial no 

- year of manufacture

- date of operation

- rated power 

- rated voltage

- rated current

- cooling type 

- vector group connection

- impedance voltge

- power frequency withstand voltge

- impulse insulation level

- no.load losses

- full load losses

- temperature rise of transformer winding

- temperature rise of oil

- mass of oil 

- total mass of transformer

Current Transformer Datasheet

Current Transformer Datasheets :

- Primary current : 100 A / 250 A / 400 A / 500 A

- Secondary current : 5 A or 1 A

- Performance (1 %) : 2.5 VA

- Accuracy class : Accuracy ±1% from 10% to 130% of rated current

- Frequency range : Frequency Range of 50 Hz to 400 Hz

- Overload capability 100 % DR Cl. 1 / Cl. 1,5 : 1.2 / 1.5

- Fault current factor / rated security factor FS : < 5

- Isolation

- Duration of rated operational voltage (bare wire)

- Input voltage max.

- Short circuit current.

- weight of current transformer

- weight of oil

Capacitor Voltage Transformer Datasheet

Capacitor Voltage Transformer Datasheet :

- manufacturer

- type

- total output simultineuos : VA

- output max : VA

- nominal system voltage : KVA

- maximum rated voltage : KVA

- performance reference voltage : V

- marked ratio

- insulation level : KV

- rated frequency : HZ

- nominal intermediate voltage : KV

- H.F Capacitance : pF

- primary capacitance C1 : pF

- secondary capacitance C2 : pF


- voltage ratio


- burden

Surge Arrester datasheet

Surge Arrester datasheet : 

- type 

- Un: Rated network voltage.   : KV

- Uc: Maximum continuous operating voltage; the maximum r.m.s. or d.c. voltage which may be continuously applied to the SPDs mode of protection. This is equal to the rated voltage. : KV

- Surge protective device (SPD): A device that is intended to limit transient overvoltages and divert surge currents. It contains at least one nonlinear component

- Test classes: Surge arrester test classification.

- In: Nominal discharge current ; the crest value of the current through the SPD having a current waveshape of 8/20. This is used for the classification of the SPD for the class II test and also for preconditioning of the SPD for class I and II tests.

- Imax: Maximum discharge current for class II test; crest value of a current through the SPD having an 8/20 waveshape and magnitude according to the test sequence of the class II operating duty test. Imax is greater than In.

- Ic: Continuous operating current ; current that flows in an SPD when supplied at its permament full withstand operating voltage (Uc) for each mode. Ic corresponds to the sum of the currents that flow in the SPD’s protection component and in all the internal circuits connected in parallel.

- Iimp: Impulse current, it is defined by a current peak value Ipeak and the charge Q. Tested according to the test sequence of the operating duty test. This is used for the classification of the SPD for class I test.

- Up: Voltage protection level; a parameter that characterizes the performance of the SPD in limiting the voltage across its terminals, which is selected from a list of preferred values. This value shall be greater than the highest value of the measured limiting voltages.
The most common values for a 230/400 V network are:
1 kV - 1.2 kV - 1.5 kV - 1.8 kV - 2 kV - 2.5 kV.

- Ures: Residual voltage, the peak value of the voltage that appears between the terminals of an SPD due to the passage of discharge current.
The SPD is characterised by Uc, Up, In and Imax

substation equipment

 substation equipment

in every electrical substation, there generaly virous indoor and outdoor switchgear equipment. each equipment has certain functional requirement :

- isolator are "disconnecting switches which can be used for disconnecting a circuit breaker. an isolator can be opened after the circuit breaker

- after opening the isolator, the earthing switch can be closed to discharge the trapped electrical charges to ground

- current transformer (CTs) and potential transformer (PTs) are used for transforming the current and the voltage to lower values for the purpose of measurement, protection,  and control

- lighting arresters ( surge arresters ) divert the over voltage to earth and protect the substation equipment from ove over voltage

calssification of circuit breaker

calssification of circuit breaker

the circuit braeker can be classified on the basisof rated voltage: circuit braeker below rated voltage of 1000 V are called low voltage circuit breaker and above 1000 V are called high voltage circuit breaker.

the type of the circuit breaker is usually identified according to the medium of arc extinction 

the classification of the circuit breaker based on the medium of arc extinction is as follows :

1- air break circuit breaker . miniature circuit breaker.

2- oil circuit breaker ( tank type or bulk oil ).

3- minimum oil circuit breaker.

4- air blast circuit breaker

5- sulphur hexafluoride circuit breaker ( single pressure or double pressure ).

Protective relay

Protective relay

Protective relay are "automatic devices which can sense the fault and send instruction to the associated circuit breaker to open" .

every part of the power system is prvided with a protective relaying system and an associated switching device.

all equipment associated with the fault clearing process are covered by the term "switchgear".

in addition to circuit breaker and protective relays, the associated equipment for controling, regulating and measuring can also be considered as switchgear devices.

switchgear includes switches,fuses,circuit breaker,isolator,relays,control panels,lighting arresters,current transformer, and various associated equipments.

switchgear is an essential part of a power system and also that of any electric circuit 

switchgear are also necessary at every switching point in the power system.

since, between the generation station and final load points, there are several voltage levels and fault levels hence, in various application, the requirements of a switchgear vary according to : 

1- location

2- rating, and

3- local requirements

circuit breaker

circuit breaker

A circuit breaker : is a switching and current-interrupting device" basically, a C.B. comprise a set of fixed and movable contacts.

the contacts can be separated by means of an operating mechanism.

the separation of current carrying contacts produce an arc.

the arc is extinguished by a suitable medium such as dielectric oil,air,vacuum,sf6 gas.

the circuit breaker serves two basic purposes :

1- switching during normal operation condition, for the purpose of operation and maintenance.

2- switching during abnormal condition, such as short circuits, and interrupting the fault currents.

disadvantage of A.C. transmission system

disadvantage of  A.C. transmission system

- the volume of copper used is much more than the D.C. system

- the inductance and capacitance of the line effects the rgulation of the line which is increased

- due to skin effect the line resistance is increased which further increases the skin effect

- the A.C. transmission lines are more effective to corona than D.C. lines

- in an A.C. sytem the speed of the generator and alternators is not economical, variation of these speeds must be controlled within very low limits

- in cable the alternatin current cause sheath loss.

- the construction of the transimission lines is not so easy as in case of D.C. lines

- the alternator must be synchronized, before they are made to run in parallel

advantage of A.C. transmission system

advantage of  A.C. transmission system

- it is possible to generate high voltage as high as 33 kv as compared to 11 kv in D.C. system

- the alternating voltage can efficienctly be stepped up by transformer which is not possible in D.C. system although in D.C. high voltage can be optained by thury system which is now obsolute

- the lowering of the voltage at sub-station is easier and cheaper as compared to D.C. system using motor generator sets or rectifiers

- the transforming sub-stations are much efficient than motor generator sets used in D.C. system

- the maintenance of A.C. sub-station is easier and is cheaper

advantage of D.C. transmission system

advantage of  D.C. transmission system

- only tow conductor are used for transmission as compared to three conductor in A.C. system. and further it is also possible causing much saving in copper

- the potential stress produced on the insulating by the D.C. system is approximately 70 per cent of the A.C. effective voltage of same value

- inductance, capacitance, phase displacement and surge problems are climinated in D.C. transmission

- no stabilizers are required when transmitting over longer distance 

- the economical use of underground cable is possible, since the stress on cable insulating is much less

neutral solidly earthing

 neutral solidly earthing

solidly earthed system are characterised by high level of fault current.

high value earth faults can be cleared quickly they can also lead to damage that is dificult to repair

solidly earthed system provide the best control of transient and temporary overvoltage that can arise btween earth and the electrical system..

insulation that is applied between phases and earth can be rated based on the phase to earth voltage.

lower rated insulation can reduce the cost of electrical system and equipment.

isolated system have one big advantage they continue operating in the presence of a single earth fault there is because thee is no rturn path available for the flow of earth fault current hence protective devices will not operate.

isolated system also have  big disadvantage transient temporary and permanent overvoltage can easly occure on such system, stressing insulation.

insulation that is applied between phase  and earth must be rated based on the phase and phase voltage, and often for even higher voltages.

the every 6 months transformer preventive maintenance

 the every 6 months transformer preventive maintenance

- pressure test for sealed tarnsformer ( look for leakage in nitrogen pressure )

- oil test take sample and checkgas in oil" analysis

- ground resistance test ( check protective relay ensure good grounding )

- check calibration to ensure working as the best

- check  all main connection cables

- infra-red check look for hot spots, loose connection damage porcelains

- check fire control ( make sure it operates correctly ).

- bushing cleaning (if the bushing have been covered in protective grease, clean and replace if in heavy polluted area the treatment by poweder blast cleaning ).

the every 3 months transformer preventive maintenance

the every 3 months transformer preventive maintenance 

- oil tests,if the transformer is operating at overload or continuous full load take sample

- gas-in-oil analysis, if condition are as above

- check pressure relief on external surface unusual dirt-moisture corrosion, overhating condition

the monthly transforemr preventive maintenance

the monthly transforemr preventive maintenance

check all gauges and bushing.

check for leak from all parts of the unit ( tank valves cooler ).

check protection alarm for proper alarm.

check ( pressure/vacuum ) gauge with manufacturer's recommendation.

check breathers ( make sure that they are free of moisture ).

the weekly transformer preventive maintenance

the weekly transformer preventive maintenance

- check oil liquid level gauge on tank & on oil filled bushing.

- for leaks and change in oil level and determine why

- check radiators by touching, cooling tubes tank waals.(look for excessive heating)

- listen for unusual noise( arcs to tanks wall can indicate dielectric failure)

the dailly transformer preventive maintenance

the dailly transformer preventive maintenance

- record load ( load current & voltage ) this record used to indicate overloading and unusual voltage effect

- check liquid level gauge to insure within safe limits

- check liquid & winding temperature indicator

- check pressure relief diaghram device ( cracked or broken must be replaced )

- looking and listening for unusual noise ( cooling fan, pump an record unusual change )

the transformer oil and state characteristcs

the transformer oil and state  characteristcs

characteristcs : limit

sludge value ( max ) : 0.10%

acidity after oxidation ( max ) : 0.40 mg KOH/g

flash point (closed) ( min ) : 140 °C

viscosity at -50c ( max )  : 800 CST

viscosity at -200c ( max ) : 40 CST

Pour Point (Max.) : -6 °C

electric strength ( breakdown ) ( min )

oil delivered in bulk : 30 kv

oil received in graet britain in drums : 27 kv

oil shipped overseas in drums : no set limit

acidity ( neutralization value ) ( max ) : 0.03 mg KOH/g

corrosive sulphur shallbno scorrosive water content ( max )

oil delivered in bulk : 35 kv

oil received in great britain in drums : 50 kv

oil shipped overseas in drums : no set limit

density at 200c ( max ) : 0.895 g/crm³

loss tangent at 900c  ( max ) :  0.005

resistivity  : no set limit

the means of cooling transformer

 the means of cooling transformer

heat is produced in normal transformer operating by copper, core and stray losses

this heat must be removed to prevent overheating of the insulating and a subsequent failure.

in the case of liquid-filled transformer the cooling is accomplished by :

1- passing the heating oil or synthetic liquid insulation over an air-coled surface by eithe natural or forced ( pumps ) convection

2- passing the heated oil over surfaces cooled by water; these surfaces being water-cooled coils installed either in the trnsformer tank or external from the tank .

all modern water-cooled transformer have external coolers.

in the case of transforemr which do not use oil or liquid as a cooling medium, cool air from the atmosphere is blown over the warm core and windings to remove the heat

Electrical protection systems

 Electrical protection systems

all those deviaces and components needed to detect and isolate a faulted system element including instrument transformer but excluding the power switch gear

-  protection relay

that electronic or electro-mechanical devices that decides if ther is a fault and then initiates action to clear that fault

- protection zone

- that portion of the power system or system element protectedby a particular protection system as defined by the location of sensing devices, usually CTs

- power system zone
a system element or group of elements protected by a particular protection system as defined by the location of the isolating device, usually circuit breaker

- unit protection
a protection system that has a closed prtection zone
that is, CTs defined the exact boundaries of the protection zone
A differential protection is an example of this type of protection

- non unit protection

a protection system that has defined boundary at the measuring location but the remote boundary depends on the setting of the measuring device, the protection relay
An overcurrent protection or a distance protection is an example of this type of protection

types of neutral earthing

types of neutral earthing 

electrical systems are usually earthed via their star point or neutral ther are three choices : 

1- neutral solidly earthed

2- neutral earthed via impedance usually aresistor in the uk

3- isolated

on the systems have no neutral, special three phase transformer or reactors may be used to artificialy derive a neutarl

Grounding of generator shaft

why the generator shaft kept grounded ? 

THE MAGNETIC OUT OF BALANCE ‎VOLTAGES AND STATIC CHARGES ARE ‎ALWAYS PRESENT ON THE SHAFT. IF THE ‎SHAFT IS NOT GROUNDED THE SHAFT ‎VOLTAGES CAN LEAD TO FORCE A VERY ‎HIGH CURRENT THROUGH THE BEARING ‎AND IT CAN RESULT IN TO THE PITTING OF ‎THE BABBIT MATERIAL.‎

Transformer cooling

Transformer cooling

 a- internal cooling medium

O : Oil

L : Insulating liquid ( nonflammable )

b- external coolind medium 

A :  air

W : water

c- type of circulation

N : natural

F : forced

d-  designation and order of symbols

1st letter :  internal cooling medium

2nd letter : type of circulation

3rd  letter : external coolind medium 

4th letter : type of circulation

 e- standard type designations

the standard types of transformers with their designation shall be as follow : 

 oil-immersed natural cooling                                                                                  ONAN

 oil-immersed water cooling                                                                                    ONWN

 oil-immersed forced-air cooling                                                                              ONAF

 oil-immersed forced-oil water cooling                                                                     OFWN

 oil-immersed forced-oil forced-air cooling                                                                OFAF

 oil-immersed forced-oil natural cooling                                                                     OFAF

 nonflammable liquid-immesed natural cooling                                                            LNAN

 nonflammable liquid-immesed water cooling                                                              LNWN

 nonflammable liquid-immesed forced-air cooling                                                         LNAF

 nonflammable liquid-immesed forced-liquid water cooling 

 nonflammable liquid-immesed forced-liquid forced coolling

 nonflammable liquid-immesed forced-liquid natural coolling

Parallel transformer

Parallel transformer

there are many conditions to connect tow transformer in Parallel :

1- same voltage input and same voltage output

2- same connection

3- same tap changing point

4- same frequency

5- same vector group

Automatic Voltage Regulator

Automatic Voltage Regulator : 

variation in generator terminal voltage is automatically regulated by variation in field current or excitation cureent

Classification of Transformers

Classification of Transformers

according to number of phases :

1- single phase transformer

2- poly phase transformer

according to frequency : 

1- very low frequency transformer

2- audio frequency transformer

3- high frequency transformer

4- IF frequency transformer

 according to turns ratio:

1- stepup transformer

2- stepdown transformer

according to thier function :

1- power transformer

2- distribution transformer

3- voltage transformer

4- current transformer