Pilot bus or weak bus is defined as the bus which, when
supported, improves voltage profile at all the buses and also ensures
additional security to the system, in terms of increased loading margin.
Usually, placing adequate reactive power support at the weakest bus enhances
static voltage stability margins. The bus which is close to experience voltage
collapse is the weakest bus. Changes in voltage at each bus for a given change in
system load is available from the tangent vector, which can be readily obtained
from the voltage collapse proximity index prediction index (VCPI) is calculated
at every bus. The value of the index determines the proximity to voltage
collapse at a bus.
The technique is derived from the basic power flow equation,
which is applicable for any number of buses in a system. The power flow
equations are solved by Newton Raphson method, which creates a partial matrix.
By setting the determinant of the matrix to zero, the index at bus k is written
as follows:
Vm is the phasor voltage at bus m
Vk is the phasor voltage at bus k
Ykm is the admittance between bus k and m
Ykj is the admittance between bus k and j
k is the monitoring bus
m is the other bus connected to bus
N is the bus set of the system.
By finding the VCPI index we can find the weak bus and weakest bus. Thus for these weak bus and weakest bus we are implementing the FACT device like STATCOM and TCSC.
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