Siteques25mar.docx

A corona ring is a conducting ring mounted on the end of or radially outside the end of an insulator in an extra high voltage system, (typically 200 kV+) The role of the corona ring is to eliminate or reduce ionisation of the air with associated corona discharge. Corona discharge damages insulators and may produce breakdown products which can cause catastrophic insulation failure. A corona ring works by modifying the shape of the electric field intensity so that the worst-case rate of change of field on the insulator is reduced, thereby reducing the peak potential across surrounding air to below the breakdown voltage of about 10 kV per inch (dry air, mains frequencies).

The non-uniform distribution of electric current over the surface or skin of the conductor carrying a.c is called the skin effect. In other words, the concentration of charge is more near the surface as compared to the core of the conductor. The ohmic resistance of the conductor is increased due to the concentration of current on the surface of the conductor. Let us consider the conductor is made up of a number of concentric cylinders. When a.c is passed in a conductor, the magnetic flux induces in it. The magnetic flux linking a cylindrical element near the center is greater than that linking another cylindrical element near the surface of the conductor. This is due to the fact that the center cylindrical element is surrounded by both the internal as well as the external flux, while the external cylindrical element is surrounded by the external flux only. The self-inductance in the inner cylindrical element is more and, therefore, will offer a greater inductive reactance than the outer cylindrical element. This difference in the inductive reactance gives a tendency to the current to concentrate towards the surface or skin of the conductor.

The effect in which the voltage at the receiving end of the transmission line is more than the sending voltage is known as the Ferranti effect. Such type of effect mainly occurs because of light load or open circuit at the receiving end.

Ferranti effect is due to the charging current of the line. When an alternating voltage is applied. Capacitance and inductance are the main parameters of the lines having a length 240km or above. On such transmission lines, the capacitance is not concentrated at some definite points. It is distributed uniformly along the whole length of the line. When the voltage is applied at the sending end, the current drawn by the capacitance of the line is more than current associated with the load. Thus, at no load or light load, the voltage at the receiving end is quite large as compared to the constant voltage at the sending end.

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Earthing of each tower is to be done after the foundation has been casted. The earthing connection which was fixed to the stub during concreting of the chimney and taken out horizontally below the ground level is used for earthing. (The installation of the earthing shall be done in accordance with IS : 5613 – 1989 (Part 3 / Section 2) for 400 kV lines or IS : 5613 – 1985 (Part 2 / Section 2) for 220 kV and 132 kV lines.)

The pipe type earthing is generally provided outside the base of the tower. A hole of the required diameter and depth is augured in the earth for the earthing pipe. The earthing pipe is then put inside the hole. A mixture of coke and salt is filled in the hole in which the earthing pipe is provided. The earthing strip which was fitted to the stub of the tower leg is then connected to the earthing pipe. The Railway authorities specify that the size of the pipe used for earthing should be of 38 mm diameter. Therefore, for towers on both sides of the Railway crossing, 2 pipes connected together are to be used for earthing. In case of difficult locations, the pipe may be laid horizontally or slanting and within the tower base or foundation pit.

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Counterpoise earthing consists of four lengths of galvanized steel stranded wires, each fitted with a lug for connection to the tower leg at one end. Galvanized steel stranded wire of the size given below is used for this purpose. a) For 400 kV lines : 7 / 3.66 mm b) For 220 kV and 132 kV lines : 7 / 3.15 mm  The wires are connected to each of the legs and taken radially away from the tower and embedded horizontally below ground level. The depth of burial below ground level and the length of each wire are normally kept according to the values given in the table below. However, the length of each wire may be increased if the resistance requirements are not met.

No. 1 2

Voltage Level of Line 400 kV 220 kV & 132 kV

Depth of Burial 1000 mm 450 mm

Length of Each Wire 25 metres 15 metres

 The tower footing resistance of all towers shall be measured in dry weather after their erection and before the stringing of earthwire. In no case the tower footing resistance shall exceed 10 ohms. In case the resistance exceeds this value, multiple pipe earthing or counterpoise earthing shall be adopted in accordance with the relevant procedure given above.  4.2 The additional earthing shall be done without interfering with the foundation concrete even though the earth strip / counterpoise lead remains exposed at the tower end.  4.3 The connections in such cases shall be made with the existing lattice member holes on the leg just above the chimney top.

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Three phase shunt reactor is generally connected to 400KV or above electrical bus system for capacitive reactive power compensation of the power system and to control dynamic over voltage occurring in the system due to load rejection. o The shunt reactor should be capable of withstanding maximum continuous operating voltage (5% higher the rated voltage in case of 400 KV system) under normal power frequency variation without exceeding the top spot temperature of 150oC at any part of the shunt reactor. Current Limiting Reactor is a kind of Series Reactors. Series Reactors are connected in the system in series. They are normally used to limit the fault current in the system or to facilitate proper load sharing in a parallel power network. When a series reactor is connected with alternator, we refer it is Generator Line Reactor. This is to minimize the stresses during three phase short circuit fault. Series reactor may also be connected in series in the feeder or electrical bus to minimize the effect of short circuit fault at other parts of the system. As effect of short circuit current in that portion of the system becomes limited, the short circuit current withstand rating of the equipment and conductors of that portion of the system can be smaller. This makes the system cost effective. When a capacitor bank is switch on in uncharged condition there may be a high inrush current flowing through it. To limit this inrush current reactor is connected in series with each phase of the capacitor bank. The reactor used for this purpose is known as damping reactor. The wave trap connected in series with the feeder line is kind of reactor. This reactor along with Coupling Capacitor of the line creates a filter circuit to block the frequencies other than power frequency. This type of reactors is mainly used to facilitate Power Line Carrier Communication. This is called Tuning Reactor. As it is used to create filter circuit, it is also called filter reactor. Commonly and popularly it is known as Wave Trap. In delta connected power system, star point or neutral point is created by using zigzag star connected 3 phase reactor, called earthing transformer. This reactor may have secondary winding to obtain power for auxiliary supply to the substation. That is why, this reactor is also referred as earthing transformer. The reactor connected between neutral and earth to limit single phase to earth fault current is called Arc Suppression Reactor. Reactor is also used to filter out harmonics present in DC power. Reactor used in DC power network for this purpose is called smoothing reactor.

Capacitors in a power distribution system are used to correct the “rective power”. The bane of AC transmission is that when the current and voltage are sufficiently out of phase, the amount of power that can be transmitted through transmission lines decreases dramatically. The most usual cause for the phase shift is inductive loads i.e. electric motors in appliances like refridgerators, air conditioners etc. The capacitors counterract the inductive loads and hence allows more load to pass through the transmission lines. It’s cheaper to connect a few capacitors than it is to build more distribution capacity. Coupling capacitors are used to transmit communication signals to transmission lines. Some are used to measure the voltage in transmission lines. In signal

transmission the coupling capacitor is part of a power line carrier circuit as shown in the schematic below. A coupling capacitor is used in this circuit in conjunction with a line trap. Line traps can be installed at the substation or on a transmission line tower.