A bus is a node, across at which one or many lines, one or many loads, and generators are connected.
In a power system, each node or bus is associate with 4 quantities, as the
- The magnitude of voltage,
- The phase angle of voltage,
- Active power,
- Reactive power.
In the load flow, problem two out of these 4 quantities are specified, and the remaining 2 are required to be determined through the solution of the equation.
CLASSIFICATION OF BUSES
Depending on the quantities that have been specified, the buses are classified into 3 categories.
For load flow studies it is assumed that the loads are constant and they are defined by their real and reactive power consumption.
The main objective of the load flow is to find the voltage magnitude of each bus and its angle when the powers generated and loads are pre-specified. To facilitate this we classify the different buses of the power system shown in the chart below.
Basically three types of buses are
A bus where there is the only load is connected and no generation exists is called a load bus. At this type of bus, real and reactive load demands Pd and Qd are drawn from the supply.
The demand is usually estimated or predicted as in load forecast or metered and measured from instruments. At sometimes, the reactive power is calculated from real power demand with an assumed power factor.
A load bus is also known as a P, Q bus. Since the load demands Pd and Qd are well known values at this bus. The other two unknown quantities at a load bus are voltage magnitude and its phase angle at the bus. In a power balance equation, Pd and Qd are treated as negative quantities since generated powers Pg and Qg are assumed positive.
A Generator bus or a voltage-controlled bus is any bus in the system where the voltage magnitude can be controlled. The real power developed by a synchronous generator can be altered by changing the prime mover input.
This in turn changes the machine rotor axis position with respect to a synchronously rotating or reference axis or a reference bus. In other words, the phase angle of the rotor 8 is directly related to the real power generated by the machine.
The voltage magnitude on the other hand is mainly influenced by the excitation current in the field winding. Thus at a generator bus, the real power generation Pg and the voltage magnitude /Vg/ can be specified. It is also possible to produce vars consumed and then contribute to voltage control. At a generator bus or voltage-controlled bus, also called a PV bus, the reactive power Qg and 8g are the values that are not known and are to be computed.
As power flows from the generators to loads through transmission lines, power loss takes place due to the losses in the line conductors. These losses when included, we get the power balance relations
Where Pg and Qg are the total real and reactive generations,
Pd and Qd are the total real and reactive power demands,
and PL and QL are the power losses in the transmission network.
The values of Pg, Qg, Pd, and Qd are either known or estimated. Since the flow of currents in the various lines in the transmission lines is not known in advance, PL and QL remain unknown before the analysis of the network.
But, these losses have to be supplied by the generators in the system. For this purpose, one of the generators or generating bus is specified as “slack bus” or “swing bus.”
At this bus, the generation Pg and Qg are not specified. The voltage magnitude is specified at this bus. Further, the voltage phase angle 8 is also fixed at this bus.