General Introduction of Electrical Sub-Station arrangment and maintenance

 Maintenance, erection and Operation of An Electrical substations

Importance:

The EHT Electrical sub-Stations are most important to transmit the power by transmission line to the load center without more deviation of the quality of the Power supply i.e to avoid voltage drop, Power loss due to high currents.

Aim:

Transmission of the power supply from generating stations to Load centers for long distances also without much voltage drop and power loss, with optimum quality to the consumer.

Requirement:

The LT voltage useful at consumer end is 3Ø 415 to 440V  or 1Ø 220 to 250V, But the generating stations are generating the power with 3h. 11KV which is to be transmitted to the far away to load centers. Due to high load current of the 11KV/440V Sub-Stations directly under the generating stations, the I²R i.e heat loss is more and voltage drop is very high. The consumer end voltage may drop very high level order of 40%. The devices of the consumer may not be operative. Hence the 11KV Power generated up to 400KV, to be transmitted and step down to deferent voltage levels [e.g 220KV à132KVà33KVà11KVà415 to440V] as per the distances and variation of the load or demand at various areas. Hence it is required to construct the electrical sub-stations at various voltage levels to transmit the power by lines.

Electrical sub-station: the electrical sub-station mainly consists of Transformers for step up at generating station and step down the voltage at load centers and in between. The transformers feeds the number of feeders with secondary voltage with the LV current=Ratio XHV current. Here Mainly Transformers step down the voltage without much change its input power to output power and frequency with small transformer losses i.e about 0.3% of copper and Iron or no load loss out of its input power.

**[Important note: The DC voltage transmission is not possible by any transformer as it does not works with zero frequency. The transformer works with the principle of Faraday's loss of electromagnetic induction i.e rate of change of electromagnetic flux which not possible in DC i.e without frequency.]

Brief explanation of EHT Sub-stations [220/132KV and 132/33K Sub-Stations

EHV (Extra High Voltage) Sub-Station forms an important link between Transmission network and Distribution network. It has a vital influence of reliability of service. Apart from ensuring efficient transmission and Distribution of power, the sub-station configuration should be such that it enables easy maintenance of equipment and minimum interruptions in power supply. Flexibility for future expansion in terms of number of circuits and transformer MVA Capacity also needs to be considered while choosing the actual configuration of the substation.EHV Substation is constructed as near as possible to the load centre. The voltage level of power transmission is decided on the quantity of power to be transmitted to the load centre. Generally, the relation between EHV Voltage level and the power to be transmitted is as follows:

1)                  Up to 80MVA to 132KV/33KV.2) From 100MVA to 300MVA 220KV/132KV.) 300 MVA to 1000 MVA 400 KV/220KV.

*SIGNIFICANT NOTES TO BE  CONSIDERED  DURING  LAYING OUT OF THE EHT  SUBSTATION*

Substations are important part of power system. The continuity of supply depends to a considerable extent upon the successful operation of sub-stations. It is, therefore, essential to exercise utmost care while designing and building a substation. The following are the important points which must be kept in view while laying out a sub-station

It should be located at a good extendable site for future expansions as the demand increases day by day. As far as possible, it should be located at the centre of gravity of load.

It should provide reliable and safety arrangement likes firefighting equipment, first aid arrangements, hospitality. For safety, consideration must be given to the maintenance of proper clearances between HT element to ground and other equipment with highly solid earthing of the body of the EHT equipment. The Facilities for carrying out repairs and maintenance, abnormal occurrences such as possibility of explosion or fire etc. for reliability must be given for good design and construction, and the provision of suitable protective switch gear etc.

·         It should be easily operated and maintained.

·         It should involve low capital cost.

·         It should be out of polluted area as the metal parts like supporting CPL and tower structures rust, dielectric level of the insulators may decrease.

·         The site should be selected for future expansion as every year the load demand increases by 15% of the previous year demand.

·         The transportation facility like road, rail should consider as heavy electrical equipment like power transformers and breakers to be transported to the site and repair centers during the construction and replacement of the failed equipment.

·         The site should suitable for using optimum floor area.

      Schematic Diagram of 132KV/33KV Sub-Station 

conclusion

Transmission and distribution stations exist at various scales throughout a power system. In general, they represent an interface between different levels or sections of the power system, with the capability to switch or reconfigure the connections among various transmission and distribution lines. The major stations include a control room from which operations are coordinated. Smaller distribution substations follow the same principle of receiving power at higher voltage on one side and sending out a number of distribution feeders at lower voltage on the other, but they serve a more limited local area and are generally unstaffed. The central component of the substation is the transformer, as it provides the effective interface between the high- and low-voltage parts of the system. Other crucial components are controlling panels, Relays, circuit breakers and instrument transformers act as protective devices that open the faulty section of the circuit automatically in the event of a fault, that is, when a protective relay find excessive secondary current according to the primary current of the current transformer connected in EHT line or equipment. An important difference between circuit breakers and switches is that breakers are designed to interrupt abnormally high range of currents i.e. from 0Amps to few Kilo Amps, whereas regular switches are designed to be operable under no loads at high voltage levels ­>11KV, and with small currents at voltage levels <11KV.  Breakers are placed on both the high- and low-voltage side of transformers to arrange internal and external protection of the transformers. . Finally, substations may also include capacitor banks to provide lagging Mvar control further small voltage improvement.

Continue with explanation of all equipment erection, maintenance, precautions and safety.