Electric power supply system in a country comprises of generating units that produce electricity; high voltage transmission lines that transport electricity over long distances; distribution lines that deliver the electricity to consumers; substations that connect the pieces to each other; and energy control centers to coordinate the operation of the components.
Transmission and Distribution Lines
- The power plants typically produce 50 cycle/second (Hertz), alternating-current (AC) electricity with voltages between 11kV and 33kV. At the power plant site, the 3-phase voltage is stepped up to a higher voltage for transmission on cables strung on cross-country towers.
- High voltage (HV) and extra high voltage (EHV) transmission is the next stage from the power plant to transport A.C. power over long distances at voltages like; 220 kV & 400 kV. Where transmission is over 1000 kM, high voltage direct current transmission is also favored to minimize the losses.
- Sub-transmission network at 132 kV, 110 kV, 66 kV or 33 kV constitutes the next link towards the end user. Distribution at 11 kV / 6.6 kV / 3.3 kV constitutes the last link to the consumer, who is connected directly or through transformers depending upon the drawn level of service.
Cascade Efficiency
- The primary function of transmission and distribution equipment is to transfer power economically and reliably from one location to another.
- Capacitors are used to correct power factor by causing the current to lead the voltage. When the AC currents are kept in phase with the voltage, operating efficiency of the system is maintained at a high level. Circuit-interrupting devices are switches, relays, circuit breakers, and fuses. Each of these devices is designed to carry and interrupt certain levels of current.
- Making and breaking the current carrying conductors in the transmission path with a minimum of arcing is one of the most important characteristics of this device. Relays sense abnormal voltages, currents, and frequency and operate to protect the system.
- Transformers are placed at strategic locations throughout the system to minimize power losses in the T&D system. They are used to change the voltage level from low-to-high in step-up transformers and from high-to-low in step-down units.
- The power source to end-user energy efficiency link is a key factor, which influences the energy input at the source of supply. If we consider the electricity flow from generation to the user in terms of cascade energy efficiency, typical cascade efficiency profile from generation to 11 – 33 kV user industry will be as below:
Single line representation of power system
Trying to represent a practical power system where a lot of interconnections between several generating stations involving a large number of transformers using three lines corresponding to R, Y, and B phase will become unnecessary clumsy and complicated. To avoid this, a single line along with some symbolical representations for generator, transformers substation buses are used to represent a power system rather neatly.
Distribution system
- Till now we have learnt how power at somewhat high voltage (say 33 kV) is received in a substation situated near load center (a big city). Loads of a big city are primarily residential complexes, offices, schools, hotels, street lighting etc.
- These types of consumers are called LT (low tension) consumers. Apart from this, there may be medium and small scale industries located in the outskirts of the city. LT consumers are to be supplied with single phase, 220 V, 40 Hz. We shall discuss here how this is achieved in the substation receiving power at 33 kV.
Power receive at a 33 kV substation is first stepped down to 6 kV and with the help of underground cables (called feeder lines), power flow is directed to different directions of the city. At the last level, step down transformers are used to step down the voltage from 6 kV to 400 V. These transformers are called distribution transformers with 400 V, star connected secondary.
Transmission lines are the most essential part of power transmission systems. Transmission of bulk power can be accomplished either by alternating current (ac) or direct current (dc), using overhead lines, or underground cables. A transmission line is characterized by four parameters:
- Series branch resistance
- Series branch inductance
- Shunt branch capacitance
- Shunt branch conductance
The performance of the transmission line depends on these parameters.
The shunt capacitance increases with an increase in the magnitude of the operating voltage. The series resistance and the shunt conductance are the least important parameters as their effect on the transmission capacity is relatively very less. However, the series resistance completely determines the real power transmission loss and hence, its presence must be considered. The shunt conductance accounts for the resistive leakage current. The leakage current mainly flows along the insulator strings and ionized pathways in the air, and changes with change in the weather, atmospheric humidity, pollution, and salt content. The effect of the shunt conductance under normal operating conditions is usually neglected.
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