CLASSIFICATION OF POWER SYSTEM BUSES

LOAD BUS

A bus where there is only load connected and no generation exists is called a load bus. At this bus real and reactive load demand P_d and Q_d are drawn from the supply. The demand is generally estimated or predicted as in load forecast or metered and measured from instruments. Quite often, the reactive power is calculated from real power demand with an assumed power factor. A load bus is also called a P, Q bus. Since the load demands P_d and Q_d are 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 P_d and Q_d are treated as negative quantities since generated powers P_g and Q_g are assumed positive.

VOLTAGE CONTROLLED BUS OR GENERATOR BUS

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 varied b: 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 δ 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 P_g and the voltage magnitude [V_g] can be specified. It is also possible to produce vars by using capacitor or reactor banks too. They compensate the lagging or leading vars consumed and then contribute to voltage control. At a generator bus or voltage controlled bus, also called a PV-bus the reactive power Q_g and δ_g are the values that are not known and are to be computed.

SLACK BUS

In a network as power flow from the generators to loads through transmission lines power loss occurs due to the losses in the line conductors. These losses when included, we get the power balance relations

Pg-P_d–P_L=O ---- (1)

Qg-Q_d-Q_L=0 ---- (2)

Where Pg and Qg are the total real and reactive generations, P_d and Q_d 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 and P_d and Q_d are either known or estimated. Since the flow of currents in the various lines in the transmission lines are not known in advance, P_L and Q_L remains 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 δ is also fixed at this bus. Generally it is specified as 0° so that all voltage phase angles are measured with respect to voltage at this bus. For this reason slack bus is also known as reference bus. All the system losses are supplied by the generation at this bus. Further the system voltage profile is also influenced by the voltage specified at this bus.

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