Repulsion Type MI instrument

Repulsion principle is very commonly used in MI instruments.The operation of such an instruments depends upon the mutual repulsion of two vanes of ferromagnetic material magnetize by a current-carrying coil.Fig shows a typical form of a repulsion-type MI instrument.It consists of a fixed coil C in which are placed two vanes A and B.Vane A is fixed in space while vane B is attached to a spindle S carrying a pointer PR.When the current to be measured is passed though the coil a magnetic field is produced.This magnetic field magnetizes the two vanes with the same polarity.Hence the vanes repel each other,with the result that the pointer is deflected.

The deflection depends on the product of the pole strengths of A and B.It is therefore proportional to the square of the magnetic field strength.Since both A and B are magnetized by the same coil current,the deflection is proportional to the square of the current.It is assumed that A and B are not magnetically saturated.Since the polarity of both the vanes reverse simultaneously,the deflection of the pointer is in the same direction for both a.c. and d.c.If a.c. is passed through the coil the deflecting torque is proportional to the mean value of the square of the current.The scale is calibrated to indicate the square root of this value for d.c. and the r.m.s. value for a.c.   

Attraction type MI instrument

The attraction type of MI instrument depends on the attraction of an iron vane into a coil carrying current to be measured. fig 1 shows a MI instruments of attraction type.A soft iron vane IV is attached to the moving system.when the current to be measured is passed through the coil C,a magnetic field is produced.This field attracts the eccentrically mounted vane towards it.Thus,the pointer PR,which is attached to the spindle S of the moving system is deflected.The pointer moves over a calibrated scale.

The control torque is provided by two hair springs S1 and S2 in the same way as for a PMMC instrument;but in such instruments springs are not used to carry any current.The damping torque is provided by the movement of a thin vane V in a closed sector-shaped box B.

If the current in the fixed coil is reversed,the field produced by it reverses,so the polarity induced on the vane reverses.Thus,whatever be the direction of current in the coil,the vane is always magnetized in such a way that it attracted into coil.The force of attraction is proportional to the square of the current,provided that the moving iron vane is not magnetically saturated.Hence,such instruments can be used both for direct and alternating currents.Attraction-type MI instruments are not commonly used in practice. 

System interconnection(Lines)

Two or more generating stations are interconnected by tie lines.Interconnection of generating stations has the following important advantages:

1.It enables the mutual transfer of energy from surplus zone to deficit zone economically.

2.Lesser overall installed capacity to meet the peak demand.

3.Lesser standby reserve generating capacity.

4.The size of the biggest unit is not related to the peak load of an individual system but it related to the peak load of the interconnected system.Thus,fewer but larger machines of greater efficiency are to be installed.

5.It permits the generation of energy at the most efficient and cheapest stations at every time.

6.It reduces the capital cost,operating cost of energy generated.

7.If there is a major breakdown of a generating system unit in an interconnected system there is no interruption of power supply.Similarly,when a machine is taken out of service for its scheduled maintenance and inspection,the continuity of supply is maintained.In other words,the planning of plant outages for maintenance and repair work is facilitated.

Thus,interconnection provides the best use of power resources and ensure greater security of supply.It enables overall economic generation by optimum use of high capacity economical generating plants.Interconnection between network is done either by  HVAC links or HVDC links.

An interconnected power system covering a major portion of a county's territory(or state) is called a grid.The different grids may be interconnected through transmission lines(called tie lines) to form a regional gird.When the different regional grids are interconnected,they form a national grid.   

Electric Power Systems

An electric power system has the following basic functions:

1.Generation of electrical energy economically with minimum ecological disturbance.

2.Transfer of this energy to consumers with maximum efficiency and high reliability at practically constant voltage and constant frequency.

Electric power generation system

Large amount of power is generated at the generating stations.The present trend is to install bigger size of alternators to generate large amount of power to cater the required increasing demand.The site of the power station depends upon the type of power station.The new thermal stations are being constructed at pit heads (near the coal mines) because of the higher cost of transportation of coal.Hydro-power station sites are governed by the availability of water resources.The nuclear plants are also situated remote from the centers of consumption due to safety reasons. Thus,the difficulty of getting power station sited near the consuming centers makes it inevitable to transfer    bulk of electrical energy through longer distances.Long-distance,bulk power transfer is only possible by high voltage transmission systems.Extra high voltage (EHV) and ultra high voltage (UHV) transmission systems have been developed in most of the countries for transporting energy from remote power stations.By EHV is meant the voltage above 220 kV. The voltages above 760kV are called ultra high voltages (UHV). 

picture of power station(nuclear power plant

Classification of Lines(electrically or physically)

Lines may be classified either electrically or physically.

Electrical Classification

1.Direct Current (DC) systems

(a) DC 2-wire ,1-wire earthed

(b) DC 1-wire ,earth return

(c) DC 2-wire, mid-point earthed

(d) DC 3-wire.

2.Alternating Current (AC) system 

(a) Single-phase,2-wire,1-wire earthed.

(b) Single-phase,1-wire,earth return.

(c) Single-phase,2-wire,mid-point earthed.

(d) Single-phase,3-wire

(e) Two-phase ,3-wire

(f) Two-phase,4-wire

(g) Three-phase,3-wire

(h) Three-phase,4-wire

All these system are used in practice. Every system has got its own field of application.The line economics varies with the individual  system.Direct current 2-wire,mid-point earthed system is used for transmitting large amounts of power over load distances.

Single-phase a.c.system is rarely used for transmitting purposes. Three-phase 3-wire a.c.system finds its greatest use for transmission and primary distribution.For secondary distribution work three-phase,4-wire system is common.The industrial and other big loads are supplied by three-phase,4-wire system.The domestic and other small load consumers are supplied with single-phase power obtained from the individual phases of a three-phase supply.

Physical Classification

The other criterion of classification  of lines is their physical configuration.The lines may be overhead,underground or underwater.Again,overhead lines may either have bare conductors or insulated conductors.

Most of the high voltage and extra high voltage lines have bare conductors since these are most economical.The conductors on overhead lines are sometimes insulated for safety or other considerations.The insulated aerial cabled although costlier than the bare conductors are far cheaper than the underground cables. They  are used in densely populated urban areas at relatively low voltages or for rural distribution work at medium voltages.

For long river or sea crossing,it is not possible to erect overhead lines due to their longer spans.In such cases the need of underwater cables becomes inevitable.such cables are very costly to manufacture and install.  

LOCAL AREA NETWORK(LAN) & WIDE AREA NETWORK(WAN)

LAN:
In an industry,commercial organization or an office which has several computers in a building or a campus,need may arise to transmit data between computers.in such a situation a high-speed interconnecting network called local area network(LAN) is needed to interconnect the computers so that they can communicate with each other.There are various ways for interconnecting computers such as star,common-bus,ring type LAN,and so on.In a star type LAN there is a central controller that  coordinates all communication between computers on the LAN.The common-bus type LAN also known as Ethernet,dose not have any central controller.Rather,the control of the bus is spread among all the computers connected to the common-bus.The common-bus is spread among all the computers connected to the common-bus.The common-bus is simply a wire usually a co-axial cable to which any number of computers can be easily connected.Any computer can take over the bus to transmit data.Two computers are not allowed to transmit data at the same time.Also,in the ring type LAN,the control is distributed among all of the computers on the network.A binary  code called token is passed around the ring from workstation.All workstation are linked to form a continuous loop.A computer which wants to transmit data must possess the token.It takes the token form the bus,to prevent any other computer from transmitting.After the transmission is completed,it puts the token on the ring so that another computer which has to transmit data can receive it.
LANs are classified according to their data transfer speed,as:high speed,medium-speed and low-speed LANs. In high-speed lANs data are transmitted at the rate of 20 millon bits per second (MBps).such LANs are designed  to link mainframe computers.In medium-speed LANs data are transmitted at the rate of 3-20 Mbps.such LANs are suitable to link smaller mainframes,minis and PCs.Example of a medium-speed LAN is Ethernet.Its speed is 10 Mbps.Low-speed LANs transmitted data at 3 Mbps.They are suitable to link PCs and other workstations.

WAN: when computers/terminals are spread over a wide area,they can e connected through public or private communication system.This type of network is called wide area network(WAN).Internet is also a kind of WAN. 

example of LAN

BUSES (in computer)

Memory and I/O devices are connected to the CPU through a group of lines called a bus.These lines are meant to carry information.These are three types of buses:address bus,data us and control bus.An address bus carries the address of a memory location or an I/O devices that the CPU wants to access.The address bus is unidirectional.The data and control buses are bidirectional because the data can flow in either direction;from CPU to memory (or I/O device) or from memory (or I/O devices) to the CPU.Examples of control signals are:

fig1 shows the schematic diagram of I/O or memory connection to CPU.

Different types of bus architectures such as ISA,PCI,AGP,USB etc.