Power Transmission

Transmission System: -The system by which the electrical power transmitted from generating station to distribution system is known as transmission system. The standard generating voltage of the generator is 11 KV, 33 KV.

1.2.0    Transmission System of electric power:

                  A power transmission system is sometimes referred to colloquially as a “grid”. However for reasons of economy, the network is rarely a grid (a fully connected network) in the mathematical sense. Redundant path and lines are provided so that power can be routed from any power plant to any load center. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line. Which due to system stability consideration may be less then the physical limit of the line deregulation of electricity companies in many countries has led to renewed interest in reliable economic design of transmission network. In Bangladesh various types of Electric power transmission system present.

Single Line Diagram Power System

Primary Transmission lines: - The line between generation station’s step-up transformer and transmission sub-station step-down transformer is known as Primary transmission lines. i.e. - The electric power at 132kV/230kV is transmitted by 3-phase, 3-wire overhead system to the outskirts of the city. This forms the primary transmission.

Secondary Transmission lines: - The line between transmission sub-station step-down transformer and sub-station step-down transformer is known as Secondary transmission lines. i.e. – The primary transmission line transmitted the power of 132kV/230kV into the transmission sub-station. This station reduced to 33kV or 66kV by step-down transformers. Then this electric power is transmitted at 33kV or 66kV by 3-phase, 3-wire overhead system to various sub-station.

            Primary Distribution lines: - The secondary transmission line terminates at the sub-stations where voltage is reduced form 33 KV to 11 KV 3-phase 3-wire. The 11 KV lines run along the important road sides of the city. This forms the primary distribution lines, It may be noted that big consumers are generally supplied power at 11 KV for further handling with sub-stations.

            Secondary Distribution lines: - The electric power from primary distribution lines is delivered to distribution sub-stations (DS). The sub-station are located near the consumer localities and step down the voltage to 400 V, 3-phase, 4-wire for secondary distribution. The voltage between any two phases is 400 V and between any phase and neutral is 230 V.

1.2.1    Types of Transmission system:

Types of Transmission System

Transmission system: - The system by which the electrical power transmitted form generating station to distribution system is known as transmission system.

According to distance of transmission system is divided by three types. These are following: -

(A) Short Transmission Line: - When the length of an overhead transmission line is up to about 50 km & the line voltage is less than 20 kV. Due to smaller length and lower voltage, the capacitance effects are small and hence can be neglected. Therefore, while studying the performance of a short transmission line, only resistance and inductance of the line are taken into account.

(B) Medium Transmission Line: - When the length of an overhead transmission line is about 50 to 150 km & the line voltage is greater than 20 kV & less than 100 kV. Due to sufficient length and voltage of the line, the capacitance effects are taken into account. The distributed capacitance of the line is divided and lumped in the form of condensers shunted across the line at one or more points.

(C) Long Transmission Line: - When the length of an overhead transmission line is more than 150 km & line voltage is greater than 100 kV. For the treatment of such a line, the line constants are considered uniformly distributed over the whole length of the line rigorous methods are employed for solution.

           

According to nature of current and voltage of transmission system is divided by two types. These are following: -

(A) D.C Transmission system: - For some years past, the transmission of electric power by D.C has been receiving the active consideration of engineers due to its numbers advantages.

(B) A.C Transmission system: - Now-a-days electrical energy is almost excessively generated, transmitted & distributed in the form of A.C.

1.2.2    Grid System:

            The entire AC network is interconnected network called national grid. Even neighboring national grids are interconnected to from super grids. In a power system when all generating station line with the operation of substations called grid system of electric power.

            In the grid system of Bangladesh power Development Board, mainly two types of transmissions lines are used. These are 230 KV and 132 KV lines. Also there is another grid line of Bangladesh i.e. - 66 KV line.

1.2.3        Grid Substation:

            As in Bangladesh, we have mainly two types of grid transmission line, one 132 KV line and another 230 KV line. So we have mainly two categories grid sub-station, the total number of grid sub-stations operated as of 2003 is 87, of which 10 numbers are 230 KV and 77 numbers are 132 KV. Capacity of 230 KV grid sub-station is 3750 MVA and 132 KV is 7051.62 MVA. In FY 2004-05 the total capacity of 230 KV Grid Substations was 4375 MVA and a total of 675 MVA of new 230/132 KV transformers were added to the 230 KV Grid Substations. In this Fiscal Year a new 230/132 KV substation at Aminbazar near Dhaka has been commissioned. In 132 KV grid, during FY 2004-05 total capacity of 132/33 KV transformer has been increased from 5405 MVA to 6581 MVA.

1.3   Protection system in overhead line:

            There are several instances when the elements of a power system are subjected to over voltages. The over voltage on the power system may be causes due to many reasons such as,

                  (a) Lightning arrester.

                  (b) Circuit breaker.

                  (c) Fuse.

                  (d) Relay.

                  (e) Surge absorber.

                  (f) Earthing

1.4   Voltage surge:

            A sudden rise in voltage for a very short duration on the power system is known as a voltage surge. When lightning strikes a line the surge rushes along the line, just as a flood of water rushes along a narrow valley when the retaining wall of a reservoir at its head suddenly gives way. In most of the cases, such surges may cause the line insulators to flash over and may also damage the near by transformers, generators or other equipment connected to the line. 

1.5.0        Causes of Over Voltage:

               The over voltage on a power system may be broadly divided into two main categories:

1.5.1        Internal Cases:

                  Internal cases of over voltages on the power system are primarily due to oscillations set up by the sudden change in the circuit conditions. We shell briefly discuss the internal causes of over voltages.

                  (i) Switching surges: - The over voltages produced on the power system due to switching operations are known as switching surges.

                  (ii) Insulation Failure: - The most common case of insulation failure in a power system is the grounding of conductor which may causes over voltage in the system.

                  (iii) Arcing Ground: - The phenomenon of intermittent is taking place in line to ground fault of a 3-phase system with consequent production of transients is known as arcing ground.

                  (iv) Resonance: - Resonance in an electrical system occurs when inductive reactance of the circuit becomes equal to capacitive reactance.

1.5.2        External Cases:

                  Internal causes do not produce surge of large magnitude. Experience shows that surge due to internal causes hardly increase the system voltage to twice the normal value. Generally surge due to internal causes are taken of by providing proper insulation to the equipment in the power system. However surges due to lightning are very severe and may increase the system voltage to several times the normal value.

1.6      Lightning:

         An electric discharge between clouds and or between the charges centers of the same cloud is known as lightning.

Type lightning strokes:

                      (1) Direct stroke.

                      (2) Indirect stroke.

(1)  Direct stroke: - When a cloud is charged positively or negatively it has a tendency to discharge when if the potential between cloud and earth exceeds the break down of air it discharge to the earth it is called direct stroke. Direct stroke has two ways 

(2)  Indirect stroke: - When a cloud is on the over head line is influences oppositely growing on the overhead line by electromagnetic induction process consequently another types of charge are grown two side of the charge, and hence it is discharged through the insulations to the earth.

 

­­