Generators provide electricity to buses, and charges extract energy from buses. In the power system network, the busses are nodes, so that a voltage for each bus can be defined.

Every bus in power system is correlated with four different volumes, including real power, reactive power, voltage and phase angle of a bus.

Why there is a need for slack busses in power system?

This answer was edited.The power system model used in the power flows shows the interconnections between plants and substations, substations and transmission lines, distribution lines and loads.

## Types of buses

Load busLoad buses are also known as PQ bus. In this type of bus, the real and reactive power is scheduled, but voltage magnitude and phase angle are unknown. We can figure it out by using load flow solution through solving using different methods. In the load bus, the phase angle of voltage is not so much important.

Generator busGenerator buses are sometimes called the voltage-controlled bus or PV bus. It is a type of bus which connects the generator to its generating transformer. In these types of buses, the real power generation and the voltage magnitude are specified for the bus. The load flow equation can be solved to find the bus voltage and reactive power level. Reactive power limits are defined for generator buses.

Slack busSystem slack bus is also called a swing bus. If the phase and voltage of the bus are defined for the bus, the bus is called a slack bus. The first bus of the power system is called a slack bus because no analysis of load flow can be carried out without a slack bus. The slack bus is the load flow solution reference bus, and one generator bus is typically used for the slack bus.

The need for a slack bus is because the power system essentially has only two kinds of buses: load and generator busses. Both powers supplied by generators and load-drawn power are defined in these buses, but power loss on transmission lines is not taken into account. The total power produced in a power system is equal to the amount of power used 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 linesThe transmission line losses can only be measured if all buses are assumed to have reactive power and a real power. The power in the buses is only understood when the load flow equations are resolved. For such purposes, one generator bus does not define actual and reactive power and such a bus is called a slack bus. The slack bus is presumed to provide the reactive and real power needed for line losses. Therefore the magnitude, phase and reactive power of a slack bus are defined, and the load flow solution provided for real and reactive powers.