Power flow studies are performed to determine voltages, active and reactive power etc. at various points in the network for different operating conditions subject to the constraints on generator capacities and specified net interchange between operating systems and several other restraints. Power flow or load flow solution is essential for continuous evaluation of the performance of the power systems so that suitable control measures can be taken in case of necessity. In practice it will be required to carry out numerous power flow solutions under a variety of conditions.
NECESSITY FOR POWER FLOW STUDIES
Power flow studies are undertaken for various reasons, some of which are the following:I. The line flows
2. The bus voltages and system voltage profile
3. The effect of change in configuration and incorporating new circuits on system loading
4. The effect of temporary loss of transmission capacity and (or) generation on system loading and accompanied effects.
5. The effect of in-phase and quadrative boost voltages on system loading
6. Economic system operation
7. System loss minimization
8. Transformer tap setting for economic operation
9. Possible improvements to an existing system by change of conductor sizes and system voltages.
For the purpose of power flow studies a single phase representation of the power network is used, since the system is generally balanced. When systems had not grown to the present size, networks were simulated on network analyzers for load flow solutions. These analyzers are of analogue type, scaled down miniature models of power systems with resistances, reactances, capacitances, autotransformers, transformers, loads and generators. The generators are just supply sources operating at a much higher frequency than 50 Hz to limit the size of the components. The loads are represented by constant impedances. Meters are provided on the panel board for measuring voltages, currents and powers. The power flow solution in obtained directly from measurements for any system simulated on the analyzer.
With the advent of the modern digital computers possessing large storage and high speed the mode of power flow studies have changed from analog to digital simulation. A large number of algorithms are developed for digital power flow solutions. The methods basically distinguish between themselves in the rate of convergence, storage requirement and time of computation. The loads are generally represented by constant power.
Network equations can be solved in a variety of ways in a systematic manner. The most popular method is node voltage method. When nodal or bus admittances are used complex linear algebraic simultaneous equations will be obtained in terms of nodal or bus currents.
However, as in a power system since the nodal currents are not known, but powers are known at almost all the buses, the resulting mathematical equations become non-linear and are required to be solved by interactive methods. Load flow studies are required for power system planning, operation and control as well as for contingency analysis. The bus admittance matrix is invariably utilized in power flow solutions.
CONDITIONS FOR SUCCESSFUL OPERATION OF A POWER SYSTEM
There are the following:1. There should the adequate real power generation to supply the power demand at various load buses and also the losses
2. The bus voltage magnitudes are maintained at values very close to the rated values.
3. Generators, transformers and transmission lines are not over loaded at any point of time or the load curve.