Power System Structure

POWER SYSTEM STRUCTURE

An electric power system consists of three main divisions namely, 

1. Generating stations

2. Transmission lines

3. Distribution system.

"Generating station" and a "distribution system" are connected through "transmission lines," which also connect one power system (grid area) to another. A distribution system connects all the loads in a particular area to the transmission lines. For economical and technological reason, individual power system are organized in the form of electrically connected areas are regional grid (also called power pools). Each area are "regional grid" operates technically and economically independent, but these are eventually interconnected to form a national grid. So that each area is contractually tied to other areas in respect to certain generation and scheduling features.

The setting of hydro station is determined by the natural water power sources. The choice of site for coal fired thermal station is more flexible. The following alternative are possible:

1. Power station may be built close to coal mines and electric energy is evacuated our transmission line to the load centers.

2. Power station may be built close to the load centers and coal is transported to them from the mines by rail road. In practice however, power station sitting will depend upon many factors technical, economical and environment.

3. As it is considerably cheaper to transport bulk energy our extra high voltage (EHV) transmission lines than to transport equivalent quantities of coal our rail road. Bulk power can be transmitted to fairly long distance of the transmission lines of 400/765 kV and above. However, the country coal resources are located mainly in the eastern belt and some coal fired station will continue to be sited in distant western and southern regions.

The Power System of today is very complex and it is a interconnected network.

The power system is subdivided into four major parts.

1. Generation

2. Transmission & Sub transmission

3. Distribution

4. Loads

1. Generations

Three phase AC Generator is one of the essential component of a Power System. It also known as alternator or synchronous generator. Synchronous generators have two synchronously rotating field.

i. One field is produced by the rotor driven at synchronous speed and excited by DC current.

ii. The other field is produced in the stator windings by the three phase armature currents.

The DC current for the rotor windings is provided by excitation system. Previously DC generators are used as exciters, which are mounted on the same shaft, providing excitation through slip rings.

Now AC generators with rotating rectifiers known as brush less excitation system are used. The generator excitation system maintains generator voltage and controls the reactive power flow. AC generators can generate high power at high voltage, nearly 30 kV. In a power plant, the size of the generator can vary from 50 MW to 1500 MW.

The mechanical input power can be given to the generators by the prime mover.

The various types are,

i. Hydraulic turbines at waterfalls.

ii. Steam turbines whose energy comes from burning of coal.

iii. Gas turbines.

iv. Internal combustion engines (some times.)

Among these prime movers hydraulic turbines operate at low speed. Due to these reason the generators, which are connected to hydraulic turbines, are a salient type rotor with many poles. Steam turbines operate at high speed. The generators used in steam turbines and cylindrical rotor two pole for 3600 rpm or four pole for 1800 rpm for operation.

Transformers

Transformers are the another major components of power system. The power transferred to secondary is almost the same as the primary and the product VI on the secondary is approximately same as the primary side.

Therefore, the turns ratio of the step up transformer 'a' will reduce the current by a ratio of 1/a. This will 'Reduce losses' in the line. The insulation requirements and other practical design problems limits the generated voltage to low value (i.e) 30 kV. In modern utility system the power may undergo four or five transformations between generator and ultimate user.

2.Transmission and Sub-transmission

The purpose of an over head transmission network is to transfer electric energy from generating units to distribution systems. Transmission lines also interconnected neighbouring utilities, which permits not only economic dispatch of power within regions during normal conditions, but also the transfer of power between regions during emergencies.

The usual generation voltage is 11 kV. For economy in the transmission of electric power, the generation voltage is stepped upto 132 kV at the generating station with the help of three phase transformers. The electric 132 kV is transmitted by three power at phase wire overhead system. This forms the "Primary transmission." The primary transmission line terminates at the receiving station. At this station, the voltage is reduced to 33 kV by step down transformers. From this station, electric power is transmitted at 33 kV by three phase three wire overhead system to various substations. This farms the "Secondary transmission".

3. Distribution

The secondary transmission line terminates at the sub-stations where voltage is reduced from 33 kV to 11 kV, three phase wire. The 11 kV lines run along all transmission lines. This forms the "Primary distribution". The electric power from primary distribution line is delivered to distribution sub stations. These sub-stations are located near the consumers, and step down the voltage to 400 V for "Secondary Distribution".

4. Loads

Loads of Power System are divided into industrial, commercial and residential. Very large industrial load may secured from the transmission system. Large Industrial loads are served directly from the sub transmission networks and small industrial loads are served from the primary distribution network.

The industrial loads are composite loads and induction motors form a high proportion of these loads. These composite loads are function of voltage and frequency and form a major part of the system load. Commercial and residential load consists of lightning, heating and cooling. These loads are independent of frequency and consume a very small reactive power. The real power of loads are expressed in terms of kilowatts of mega watts (MW).

The magnitude of load varies throughout the day and power must be available to consumer on demand.

Daily load factor =Average Load/ Peak Load

Annual load factor =Total Annual Energy/Peak Load x 8760 hr