In a power system various component are meet at buses. The generators feed energy to buses and loads draw energy from buses. In the network of a power system the buses become nodes and so a voltage can specified for each bus.
So, each bus in a power system is associated with four quantities and they are real power, reactive power, phase angle of voltage and magnitude of voltage. In a load flow problem two quantities (out of four) are specified for each bus and the other two quantities are obtained by solving the load flow equations. The buses of a power system can be classified into following three types based on the quantities being specified for the buses. The table below shows the quantities specified and to be obtained for each type of bus.
(i) Load bus (or PQ-bus)
(ii) Generator bus (or PV bus or voltage-controlled bus)
(ii) Slack bus (or swing bus or reference bus)
Types of buses
|Bus type||Quantity specified||Quantities to be obtained|
|Load bus||P, Q|||V|, ẟ|
|Generator bus||P, |V|||Q, ẟ|
|Slack bus|||V|, ẟ||P, Q|
P = PG – PD ; |V| = Magnitude of bus voltage
Q = QG – QD ; ẟ = Phase of bus voltage
PD , QG , = Real and Reactive powers drawn by the loads connected to the bus respectively.
The bus is called load bus, when real and reactive components of power are specified for the bus. The load flow equations can be solved to find the phase and magnitude of bus voltage. In a load bus the voltage is allowed to vary within permissible limits, for example ±5%.
The bus is called generator bus, when magnitude of bus voltage and real power are specified for the bus. The load flow equation can be solved to find the phase of bus voltage and reactive power. For generator buses reactive power limits will specified.
The bus is called slack bus if the phase and magnitude of bus voltage are specified for the bus. The slack bus is the reference bus for load flow solution and usually one of the generator bus is selected as the slack bus Need for slack bus.
Need for slack bus:
Basically, the power system has only two types of buses and they been load and generator buses. In these buses only power injected by generators and power drawn by loads are specified, but the power loss in transmission lines are not accounted. In a power system the total power generated is equal to the sum of power consumed by loads and losses.
Therefore, in a power system,
(sum of complex Power of generator) = (sum of complex Power of loads) + (Total (complex) power loss in transmission lines)
(Total (complex)power loss in transmission lines) = (Sum of complex sum power of generators) –
(Sum of complex power of loads)
The transmission line losses can be estimated only if the reactive power and real power of all buses are known. The powers in the buses will be known only after solving the load flow equations. For these reasons, the real and reactive power of one of the generator buses is not specified and this bus is called slack bus. It is assumed that the slack bus generates the reactive and real power required for transmission line losses. Hence for a slack bus, the magnitude and phase of bus voltage are specified and real and reactive powers are obtained through the load flow solution.
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