Methods to Decrease earth resistance

Clean the surface of the electrode, mix the soil in the earthing pit with charcoal, salt and soft soil. Pour sufficient water in the earthing pit.  Connect the earthing rod with the equipment by low resistance copper strip. Use  more number of electrodes, larger mesh.

Gas Insulated Switchgear

SF6 Gas Insulated Switchgear (GIS) are preferred for voltage rating of 36 kV, 72.5 kV, 145 kV, 245 kV, 420 kV and above. In such a substation, the various equipments like Circuit Breakers, Busbars, Isolators, Load Break Switches, Current Transformers, Voltage Transformers, Earthing Switches, etc are housed in metal enclosed modules filled with SF6 gas. The SF6 gas provides the phase to ground insulation and phase to phase insulation. As the dielectric strength of SF6 gas is higher than air, the clearance required are smaller. The overall size of each equipment and the complete substation is reduced to about 10% of convention air-insulated substation. GIS are installed indoor.

The various modules of GIS are factory assembled and are filled with SF6 gas at a pressure of about 3kgf/cm2 .  Thereafter, thy are taken to site for final assembly. Such substations are compact and can be installed conveniently on any floor of a multistoried building or in an underground substation. The installation time s substantially reduced. Such installation are preferred in cosmopolitan cities , industrial townships, hilly terrains etc., where cost of land is very high and higher cost of  gas insulated switchgear is justified by saving due to reduction in floor area requirement. They are also preferred in heavily pollutes areas where dust, chemical fumes and salt layers cause frequent flashovers in conventional outdoor air-insulated switchgear.

The SF6 Gas Insulated Switchgear (GIS) contain the same component as the conventional outdoor substations. All live parts are enclosed in metal housings filled with SF6 gas. The live parts are enclosed in metal housings filled with SF6 gas. The live parts are supported on cast resin insulators. Some of the insulators are designed as barriers between neighboring modules such that gas do not pass through them. The entire installation is sub-divided into compartments which are gas tight with respect to each other. Thereby the gas monitoring system of each compartment can be independent and simpler.

The enclosures are non-magnetic material such as aluminum or stainless steel and are earthed. The gas tightness si provided by static ‘O’ seals placed between machined flanges. The ‘O’ rings are placed in the grooves such that after assembly, the O ring get squeezed by about 20%. Quality of material and dimension of grooves and O-seals are important to ensure gas tight performance.

The GIS has gas monitoring system. The gas density in each compartment is monitored. If pressure drops slightly, the gas is automatically tapped up. With further gas leakage, the lo pressure alarm is sounded or automatic tripping or lock out occurs.

Creep in Hydro Turbines

Creep is the situation in which there is slow rotataion (below 20 rpm) due to leakage of water through wicket gates which is always there. Creep situation is totally unwanted and undesirable for the m/c life. To know its presence we usually apply an instrument known as Creep Indicator. This assembly consists of solenoid and brushes. In ususal case when unit is not runing brush has contact with shaft and for any movement of shaft, brush tilts actuating a micro switch which give an alarm signal. Creep inidcator is designed such that it remains insensitive during Start Up, Running and Stopping. During this period solenoid get energised and removes the brush contact from the shaft.

Major Components of a Hydro Power Plant

1. Concrete Dam
2. Head Race tunnel
3. Surge Shaft
4. Pressure Shaft
5. Power House
6. Penstock
7. Main Inlet valve(MIV) & Bypass Valve
8. Turbine
9. Generator
10. Cooling Water System
11. Fire Fighting System
12. Ventilation System
13. Drainage System.
14. Auxillary Supply System
15. DC Supply System
16. Transformer Gallery
17. SwitchYard
18. Tailrace Tunnel (TRT)

Advantages of A.C. over D.C. / Why generation is done in A.C.

Electric energy is generated and used is AC because it offers much advantages then DC. The few are listed below:-

1.  AC can be generated at high voltages, but DC cannot be generated at high voltages because sparking starts at the commutator at high voltage, due to which commutator gets damaged.
2. High voltages AC generators are much simpler and cheaper than DC generators of the same range. It is because in AC  generators there i no commutator which is costly part and is damaged. 
3. Alternating current can be stepped up or stepped down with a static device  called transformer. When voltages is stepped up current decreases to a small value. Small current produces less heat and can be transmitted through a thin conductor.Thus it is possible to transmit AC at high voltages. This reduces the size of conductor, transmission losses ans increases transmission efficiency. At the receiving station, voltages can be stepped down to the required value by using step down transformer. This is most important reason for generating and using electrical energy as AC.
4. A.C. induction motors are simplest in construction, cheaper in cot and require less maintenance whereas D.C. motors are complicated.