BUS Classification

In a power system each node or bus is associated with four quantities, such as magnitude of voltage, phase angle of the voltage (δ), active or true power (P) and reactive power (Q). In a load flow problem two out of these four quantities are specified and the remaining two are required to be determined through the solution of equations. Depending upon which quantities have been specified, the buses are classified into three categories.

Slack bus/Swing Bus/Reference Bus

One of the generation buses in a power system is chosen as slack or swing bus. At this bus, voltage magnitude |Vi| (usually set to 1 pu) and voltage phase angle δi (usually set to 0°). The active and reactive powers at this bus are to be determined through the solution of equations. If slack bus is not specified, the generation bus usually with maximum active power P is taken as reference bus.

PQ Bus/Load Bus

At a load or PQ bus, the net powers P_i and Q_i are known while |V_i| and δ_i are unknown. A PQ bus may be an exporting or importing bus. It comprises almost 85% of all the buses in a power system. A pure load bus has P_Gi = Q_Gi = 0, i.e., no generating facility at the bus.

PV Bus/Generator Bus

This bus is always connected to a generator. Here, P_Gi and |V_i|are specified. Hence, the net power P_i is known. The values of Q_i and δ_i are unknown at this bus. PV/Generator bus comprises of about 15% of all the buses in a power system. All PV buses can maintain a constant voltage as long as reactive power is within the limit.

Voltage Controlled Bus

The PV bus and the voltage controlled bus are grouped together. But, a voltage controlled bus differs from a PV bus by the fact that the voltage controlled bus has voltage control capabilities, and uses a tap adjustable transformer and/or a static VAR compensator instead of a generator. At these buses P_i , Q_i and |V_i| are known whiles δ_i is unknown. At a voltage controlled bus

P_Gi = Q_Gi = 0
P_i = P_Gi – P_Di and Q_i = Q_Gi – Q_Di therefore,
P_i = –P_Di and Q_i = –Q_Di

Compensation Technique

There are two types of compensation techniques :
(i) Static VAR Compensator : Consider as PV Bus.
(ii) Shunt Capacitance Inductor : Consider as PQ bus because it has constant impedance value.

Note:

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