18-08-2012, 12:40 PM
LOAD FLOW STUDY IN POWER SYSTEM
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INTRODUCTION
In a three phase ac power system active and reactive power flows from the generating station to the load through different networks buses and branches. The flow of active and reactive power is called power flow or load flow. Power flow studies provide asystematic mathematical approach for determination of various bus voltages, there phase angle active and reactive power flows through different branches, generators and loads under steady state condition. Power flow analysis is used to determine the steady state operating condition of a power system. Power flow analysis is widely used by power distribution professional during the planning and operation of power distribution system.
OBJECTIVE OF LOAD FLOW STUDY
Power flow analysis is very important in planning stages of new networks or addition to existing ones like adding new generator sites, meeting increase load demand and locating new transmission sites.
The load flow solution gives the nodal voltages and phase angles and hence the power injection at all the buses and power flows through interconnecting power channels.
It is helpful in determining the best location as well as optimal capacity of proposed generating station, substation and new lines.
It determines the voltage of the buses. The voltage level at the certain buses must be kept within the closed tolerances.
System transmission loss minimizes.
Economic system operation with respect to fuel cost to generate all the power needed
The line flows can be known. The line should not be overloaded, it means, we should not operate the close to their stability or thermal limits.
BUS CLASSIFICATION
A bus is a node at which one or many lines, one or many loads and generators are connected. In a power system each node or bus is associated with 4 quantities, such as magnitude of voltage, phage angle of voltage, active or true power and reactive power in load flow problem two out of these 4 quantities are specified and remaining 2 are required to be determined through the solution of equation. Depending on the quantities that have been specified, the buses are classified into 3 categories.
VARIABLES AND BUS CLASSIFICATION
Buses are classified according to which two out of the four variables are specified
Load bus: No generator is connected to the bus. At this bus the real and reactive power are specified.it is desired to find out the volatage magnitude and phase angle through load flow solutions.It is required to specify only Pd and Qd at such bus as at a load bus voltage can be allowed to vary within the permissible values.
Generator bus or voltage controlled bus: Here the voltage magnitude corresponding to the generator voltage and real power Pg corresponds to its rating are specified.It is required to find out the reactive power generation Qg and phase angle of the bus voltage.
Slack (swing) bus: For the Slack Bus, it is assumed that the voltage magnitude |V| and voltage phase Θ are known,whereas real and reactive powers Pg and Qg are obtained through the load flow solution.
FLEXIBLE AC TRANSMISSION
Flexible transmission system is akin to high voltage dc and related thyristors developed designed to overcome the limitations of the present mechanically controlled ac power transmission system.
Use of high speed power electronics controllers, gives 5 oppertunities for increased efficiency.
Greater control of power so that it flows in the prescribed transmission routes.
Secure loading (but not overloading) of transmission lines to levels nearer their required limits.
Greater ability to transfer power between controlled areas, so that the generator reserve margin- typically 18 % may be reduced to 15 % or less.
Prevention of cascading outages by limiting the effects of faults and equipment failure.
Damping of power system oscillations,which could damage equipment and or limit usable transmission capacity.