Friday, August 12, 2011

Elements of Power System Part2

1.3 Bus Bars
       Bus bars are the common electrical component that connect electrically number of lines which are operating at the same voltage directly. These bars are of either copper or aluminium generally of rectangular cross-section. The can be of other shapes such as round tubes, round solid bars or square tubes.
       The outdoor bus bars are of two types viz the rigid type or strain type.
       In the rigid type of the bus bars, pipes are used. The pipes are also used for making connecting among different components. The pedestal insulators support the bus bars and the connections. The equipments and bus bars are spread out and requires large space. The clearance remain constant as the bus bars are rigid.
       It has following advantages.
1) The maintenance is easy as bus bars and connections are not very high from ground.
2) As pipe diameter is large, the corona loss is less.
3) Reliability is more than strain type.
      Following are its limitations.
1) Large area is required.
2) It requires comparatively high cost.
       In strain type, bus bars are an overhead system of wires between two supporting structure and supported by strain type insulators. As per the size of the conductors, the stringing tension can be limited (500 - 900 kg).
       The advantage of this type is its economy and it is recommended presently due to general shortage of aluminium pipes.
       The material used in case of rigid type bus bars is aluminium pipes. The general sizes of pipes commonly used for voltages are as given below.
                              33 KV                            40 mm
                              66 KV                            65 mm
                              132 KV                          80 mm
                              220 KV                          80 mm
                              400 KV                         100 mm
       Due to rapid oxidization of aluminium, proper care must be taken while doing connections. In order to avoid strain of supporting insulators due to thermal expansion or contraction of pipe, joints should be provided.
        In case of strain type arrangement, material used is ACSR (Aluminium Conductors with Steel Reinforcement) and all aluminium conductors. For high rating of bus bars bundled conductors are used. The commonly used sizes are as below.
                      66 KV            37/2.79 mm                  ACSR
                      132 KV          37/4.27 mm                  ACSR
                       220 KV         61/3.99 mm                  ACSR
                       400 KV         61/4.27 mm                  ACSR in duplex
1.4 Circuit Breaker
       The circuit breakers are used to open or close a circuit under normal and faulty conditions. It can be designed in such a way that it can be manually operated or by remote control under normal conditions and automatically operated during fault. For automatic operation, relay circuit is used.
       The circuit breakers are essential as isolators can not be used to open a circuit under normal conditions as it has no provision to quench arc that is produced after opening the line. It has to perform following functions.
i) Full load current is to be carried continuously.
ii) Opening and closing the circuit on no load.
iii) Making and breaking the normal operating current.
iv) Making and breaking the fault currents of magnitude upto which it is designed for.
       Upto 66 KV voltages, bulk oil circuit breakers are used. Voltages greater than 66 KV, low oil circuit breakers are used. For still high voltages, air blast, vacuum or SF6 circuit breakers are used.
1.5 Isolators 
      In order to disconnect a part of the power system for maintenance and repair purposes, isolating switches are used. These are operated after switching off the load by means of a circuit breaker. The isolators are connected on both sides of circuit breakers. Thus to open isolators, circuit breakers are to be opened first.
       An isolator is essentially a knife switch and is designed to open a circuit under no load that is lines in which they are connected should not be carrying any current.
       Use of isolators in a substation is shown in the Fig. 1.
Fig.1 Line diagram of substation with use of isolating switches

       As shown in the Fig.1, there are 5 sections. With the help of isolators, each section can be disconnected for repair and maintenance. If it is required to do maintenance in section 4, then the circuit breaker in that section is to be opened first and then open the isolators 3 and 4. Thus section 4 is open for maintenance. After maintenance, the isolators 3 and 4 are to be closed first and then circuit breaker is closed.
       In some cases, isolators are used as circuit breaking devices. But is limited by particular conditions such as power rating of given circuit. The isolators are of two types viz single pole and three pole isolators.

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