The power system is a network which contain generation, distribution and transmission system. It involves process of converts one form of energy into electrical energy. This system includes the devices connected to the system like the synchronous generator, motor, transformer, circuit breaker, conductor, Electrical substation etc.
Power system has major six components such as power plant, transformer, transmission line, substations, distribution line, and distribution transformer. The transformer in transmission system involves the process of step down and step up power which has generator from power plant.
The suitable power to the consumers from substation where the power is step down by distribution transformer. The power from the transmission line is transfer to different Substations Electricity is generated at central power stations and then transferred to loads (i.e., Domestic, Commercial and Industrial) through the transmission and distribution system.
A combination of all these systems is collectively known as an Electric Power System. A power System is a combination of central generating stations, electric power transmission system, Distribution and utilization system.
Structure of Power system
The power system is the complex enterprise that may be subdivided into the following sub-systems.
Generating Station
In power system at generating station, generated electrical power arrange from 11KV to 25KV. This voltage may transfer to very large distance by step up transformer in transmission line. The various plant in power system at generating station are nuclear plant, thermal plant and hydro thermal plant.
In generating station, electrical power generates by converting mechanical power,Ø which getup by gas turbines, fuels like gas and nuclear, coal. The major equipment in this sation is transformer as well as generator.
Transmission Substation
The process of transmission line to supply large range of bulk power to substations and heavy loaded consumers. It takes overhead lines to transfer power from generation and carries to distribution substations.
The transmission lines carry the double major functions. It transports the energy from generating stations to bulk receiving stations.
The bulk power is received at receiving station which is carries by transmission system. It interconnects the two or more generating stations. The neighboring substations are also interconnected through the transmission lines.
The voltage at transmission line is performing at larger than 66kv and is standardized at 11kV,66kv, 132KV, 220KV, 400kV,500KV(DC), and 765KV, line-to-line. The transmission line above 220KV is usually referred to as extra high voltage (EHV).
Sub Transmission Substation
In Sub transmission system, the part of transmission system that link the large voltage substation, where the power is step down and transfer to distribution substation. Large and bulk power consumers are directly powered by sub transmission system.
It has voltage varies from 90 KV to 138KV. In substation, the voltage at transmission line is maintained by locating capacitor and reactor. The operation of the sub-transmission system is similar to that of a distribution system.
It has a larger voltage level when compare to a distribution system. It distributes large power. It distributed minor substations as balance to a distribution system which distributes few loads.
Distribution Substation
In an area, the entire consumers are connected by components in a power system to large power source is termed as distribution system. The transmission system which interconnects the large bulk power system and generating plant. It also fed some load which could be nearby it.
Types of Transmission Line Interconnection
The Transmission interconnection between network is mainly classified into two types,
HVAC link (High Voltage Alternating Current)
HVDC link. (High Voltage Direct Current)
HVAC (High Voltage Alternating Current)
In HVAC link the two AC systems are interconnected by an AC link. For interconnecting the AC system, it is necessary that there should be sufficiently close frequency control on each of the two systems.
For the 50Hz system, the frequency should lie between 48.5 Hz and 51.5 Hz. Such an interconnection is known as synchronous interconnection or synchronous tie. The AC link provides a rigid connection between two AC system to be interconnected. But the AC interconnection has certain limitations.
The interconnection of an AC system has suffered from the following problems.
- The interconnection of the two AC networks is the synchronous tie. The frequency disturbances in one system are transferred to the other system.
- The power swings in one system affects the other system. Large power swing in one system may result in frequent tripping due to which major fault occurs in the system. This fault causes complete failure of the whole interconnected system.
- There is an increase in the fault level if an existing AC system is connected with the other AC system with an AC tie line. This is because the additional parallel line reduces the equivalent reactance of the interconnected system. If the two AC system are connected to the fault line, then the fault level of an each AC system remains unchanged.
HVDC (High Voltage Direct Current)
The DC interconnection or DC tie provides a loose coupling between the two AC system to be interconnected. The DC tie between two AC systems is non-synchronous (Asynchronous).
The DC interconnection has the certain advantages.
- The DC interconnection system is asynchronous thus the system which is to be interconnected is either of the same frequency or at the difference frequency. The DC link thus provides the advantages of interconnection of two AC network at different frequencies. It also enables the system to operate independently and to maintain their frequency standards.
- The HVDC links provide fast and reliable control of magnitude and direction of power flow by controlling the firing angle of converters. The rapid control of power flow increases the limit of transient stability.
- The power swings in the interconnected AC networks can be damped rapidly by modulating the power flow through the DC tie. Thus, the stability of the system is increased.