Concept of Decoupled & Fast Decoupled Load Flow and Application of Load Flow Analysis

Decoupled load flow:

Since active power P is closely associated with power angle but it is less associated with bus voltage magnitude. An important characteristic of any practical electric power transmissions system operating in steady state is the strong interdependence between real powers and bus voltages angles and between reactive powers and voltage magnitudes .This interesting property of weak coupling between P - and Q-V variables gave the necessary motivation in developing the decoupled load flow (DLF) method, in which P−and Q-V problem are solved separately .In any conventional Newton method, half of the elements of the Jacobean matrix represent the weak coupling, and therefore may be ignored' Any such approximation reduces the true quadratic convergence to geometric one.

The main advantage of the Decoupled Load Flow (DLF) as compared to the NR method is its reduced memory requirement in storing the Jacobian. There is not much of an advantage from the point of view of speed since the time per iteration of the DLF is almost the same as that of NR method and it always takes a number of iterations to converge because of the approximation.

Fast decoupled Load flow:

Further physically justifiable simplifications may be carried out to achieve some speed advantage without much loss in accuracy of the solution using (DLF) model. The result is a simple, faster and more reliable than the (NR) method called the fast decoupled load flow (FDLF).Sub-matrices can be further simplified, using the guidelines given below to eliminate the need for re-computing of the sub-matrices during each iteration.It is a variation on Newton-Raphson that exploits the approximate decoupling of active and reactive flows in well-behaved power networks and additionally fixes the value of the Jacobian during the iteration in order to avoid costly matrix decompositions. Also referred to as "fixed-slope, decoupled NR". Within the algorithm, the Jacobian matrix gets inverted only once, and there are three assumptions. Firstly, the conductance between the buses is zero. Secondly, the magnitude of the bus voltage is one per unit. Thirdly, the sine of phases between buses is zero. Fast decoupled load flow can return the answer within seconds whereas the Newton-Raphson method takes much longer. This is useful for real-time management of power grids.

Assumptions:

i. Some terms in each element are relatively small and can be eliminated.

ii. The remaining equations consist of constant terms and one variable term.

iii. The one variable term can be moved and coupled with the change in power variable.

iv. The resultant is a Jacobean with constant term elements.

Additional assumptions to decoupled method are,

i.As the transmission line has higher reactance in comparison to resistance.so we can neglect the resistance.i.e. R=0

ii.Voltage magnitudes on each bus are near equals to 1 pu.So in some cases,voltage magnitudes can be assumed to be exactly 1pu.

iii.The difference of the phase angle between two buses is very small.

The fast decoupled load flow solution yields:

Application of load flow solutions:

The flow of active power and reactive power is called load flow.The voltage of the bus & power angles is affected by power flow and vice versa.The applications of load flow solution are listed below:

1. Calculation of line losses.
2. Knowing the effect of reactive power compensation on bus voltage.
3. Preparing a software for online operation,control & monitoring of power system.
4. Obtaining the initial input data for various other power system studies such as economic load dispatch,reactive power & voltage control.
5. It is essential for power system planning,designing,expansion design & for providing guidelines to control room.