Simulink Getting Started

       

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Definition of Nonlinear Loads

Nonlinear Load

This type of load is very much in demand all over the world, regardless whether they

are industrial, commercial, or domestic users. Computers, air conditioners, and televisions are examples of this type of load and it is understandably impossible to eliminate them away, even their impact to power system is already well documented. Nonlinear load, is define as,

“ A class of load that, when subjected to sinusoidal voltage source, would draw

 non-sinusoidal current into the system.”

	TYPE OF USER	TYPE OF LOADS	ELECTRICAL CHARACTERISTICS
	DOMESTIC COMMERCIAL INDUSTRIAL	Incandecent Lamps, Water heaters Computers, Flourocent Lamps, Water heaters. Motors, ASDs Furnances, Pumps	Resistive            Resistive            Resistive            Resistive            Nonlinear            Inductive

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Characteristics of different earthing systems:

TN system neutral-connected exposed conductive part

•    Letter T depicts that

            The neutral is directly earthed.

  

•     Letter N depicts that

            The exposed conductive parts of the loads are connected to the neutral conductor.

There are two types of systems, possibly depending on whether the neutral

conductor and protective conductor (PE) are combined or not:

 Case 1: When neutral and protective conductors are joined in a single conductor in this case it is known as PEN. This type system is identified by a third letter C and is called TNC system. PEN conductor eliminates potential rises in the exposed conductive parts if a fault occurs.

(Reference: IEC 60364-5, section 546-2) 

 Case 2: In this case the neutral conductor and protective conductor are different. These type of systems are recognized by a third letter S and is said as TNS.

Characteristics of different earthing systems:

IT system: Specific characteristics:

•     Switching upon occurrence of a double fault is usually generated by phase-to phase fault protective devices (circuit-breakers, fuses, etc.).

•     If the short-circuit current is not sufficient to activate protection against phase-2-phase faults, In        case if the loads are far away, protection should be done by residual current devices (RCD).                               

           

TT system:

Specific characteristics:

•    The installation of RCDs is compulsory.

•     All exposed conductive parts protected by the same protective device should be connected to the same earth.

•     The neutral earth and the exposed conductive part earth may or may not be Interconnected or combined.

•  The neutral may or may not be distributed.

Specific characteristics of TNS and TNC systems:

•  Fault switching is obtained by devices providing protection against phase2 phase faults (circuit-breakers, fuses, etc.).

• both TNS and TNC systems can be used in the same installation. The TNC system (4 wires), however, must never be downstream of the TNS system (5wires).

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MATLAB FOR BEGGINERS

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PROTECTION OF THE GENERATOR DUE TO UNBALANCE LOADING

Because of fault there is a unbalance condition in the three phase stator currents and because of these unbalance currents dual frequency currents starts flowing in rotor core. these dual frequency currents heats up rotor and harms rotor. due to same unbalance currents damage of stator also takes place.

 Negative arrangement channel furnished with the over current relay is utilized for the insurance against unbalance loading. From the hypothesis of the symmetrical components, we realize that unbalanced three phase currents contains the negative sequence part. This negative sequence current reasons warming of the stator.The negative warming accompanies the resistance law so it is corresponding to the square of the current. The warming time consistent normally rely on the cooling framework utilized and is equivalent to I²t=k where I is the negative succession current and t is the time of flow in seconds and k is the constant generally lies between 3 and 20. Its general practice to utilize negative current transfers which matches with the above warming aspects of the generator. In this sort of assurance three Cts are joined with three stages and the yield from the secondaries of the Cts is sustained to the curl of over current transfer through negative succession channel. Negative succession circuit comprises of the resistors and capacitors and these are joined in such way that negative succession currents moves through the transfer loop. The hand-off might be set to work at any specific quality of the unbalance currents or the negative succession part current.

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Earthing standard and Terminology (Part-1)

Earthing systems are governed by standard IEC 60364-3. There are three types of systems: IT, TT and TN.

The first letter defines the neutral point in relation to earth:

•     T stands for directly earthed neutral.

•     I stand for unearthed or high impedance-earthed neutral (e.g. 2200 Ω).

 

The second letter defines the exposed conductive parts of the electrical  Installation in relation to earth:

• T stands for directly earthed exposed conductive parts.

•    N stands for exposed conductive parts directly connected to the neutral conductor.

 

IT system unearthed or impedance-earthed neutral (see Figure-1)

Letter I depicts that

The neutral is unearthed or connected to earth by a high impedance (an Impedance of 1600 Ω is often used).

Letter T depicts that

The exposed conductive parts of the loads are interconnected and earthed. A group of loads can be individually earthed if it is situated far away from the Other loads.

TT system directly earthed neutral 

 First letter T depicts that

The neutral is directly earthed.

Second letter T depicts that

The exposed conductive parts of the loads are interconnected either altogether or by a group of loads. Each interconnected group is earthed. One exposed conductive part can be individually earthed if it is far away from the others.

Figure 2-3: directly earthed neutral (TT system) in low voltage

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Earthing and it’s Importance

In low voltage three-phase system there are three single Phases measured between each phase and a common 

point called the “Neutral point”. In actual fact, the neutral is the common point of three Star-connected windings

where all three phases meet.

Meaning of Unearthed and Earthed System:

When there is an open circuit between the neutral point and earth, we say that theNeutral is Unearthed.

On the other hand if there is a connection between earth And neutral in this case system is called Earthed.

A directly earthed neutral strongly limits over voltages but it causes very high fault currents, whereas an

unearthed neutral limits fault currents to very low values But encourages the occurrence of high over 

voltages.

In any installation, service continuity in the event of an insulation fault is also Directly related to the earthing system. An unearthed neutral permits service Continuity during an insulation fault. Contrary to this, a directly earthed neutral, or Low impedance-earthed neutral, causes tripping as soon as the first insulation fault

Occurs.

However, in an unearthed or high impedance-earthed network, the damage is Reduced, but the equipment 

must have an insulation level compatible with the level of over voltages able to cope in this type of network. The choice of earthing system in both low voltage and medium voltage networks depends on the type of

installation as well as the type of network. It is also influenced by the type of loads  and service continuity required.

From above discussion it is clear that we have to choose a compromise between The two, supply continuity and insulation protection.

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SOME IMPERATIVE SENTENCES FOR DAILY USE

There are some sentences which are very common in use with common mistakes.

        RIGHT                                                            WRONG

1.) READ OUT                                                       READ

2.) SPEAK OUT                                                     SPEAK

3.)  SHUT THE DOOR                                           CLOSE DOOR

4.) BOLT THE DOOR                                            LOCK THE DOOR

5.) MY NAME IS MIKE                                         MYSELF MIKE

6.) PLEASE SHIFT A BIT                                       PLEASE LET ME SIT

7.) SORRY                                                               WHAT

8.) I AM PURSUING MBA                                      I AM DOING MBA

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