Bus Classification in Power Systems
Demonstrative Video
Bus or Busbar
An arrangement of conductor that serves as a common connection for the conductors of two or more circuits is known as bus or busbar.
Buses are meeting points of various components.
Generators will feed energy to buses and loads will draw energy from buses.
In the network of a power system the buses becomes nodes and so voltage can be specified for each bus.
Each bus in PS is associated with four quantities:
real (\(P\)) and reactive (\(Q\)) power
magnitude (\(|V|\)) and phase angle (\(\delta\)) of voltage
Two quantities are specified and other two are to be determined
Bus Classification
For load flow studies it is assumed that the loads are constant and they are defined by their real and reactive power consumption.
It is further assumed that the generator terminal voltages are tightly regulated and therefore are constant.
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.
Load Bus (P-Q bus):
no generators are connected and hence the generated real power \(P_{G}\) and reactive power \(Q_{G}\) are taken as zero
required to specify only -\(P_L\) and -\(Q_L\) in which the negative sign accommodates for the power flowing out of the bus
The objective of the load flow is to find the bus voltage magnitude \(|V_{i}|\) and its angle \(\delta_{i}\)
Generator or Voltage Controlled Bus (P-V bus):
the buses where generators are connected
power generation is controlled through a prime mover while the terminal voltage is controlled through the generator excitation
Keeping the input power constant through turbine-governor control and keeping the bus voltage constant using automatic voltage regulator, we can specify constant \(P_G\) and \(|V|\)
\(Q_{G}\) supplied by the generator depends on the system configuration and cannot be specified in advance
Furthermore we have to find the unknown angle \(\delta_{i}\) of the bus voltage
Slack, Swing or Reference Bus: (V−δ bus)
to balance the active and reactive power in the system
provides or absorbs \(P\) and \(Q\) power to and from the TL to provide for losses, since these variables are unknown until the final solution is established
serve as an angular reference for all other buses in the system, which is set to \(0^{\circ}\)
The voltage magnitude is also assumed to be 1 p.u.
If a slack bus is not specified, then a generator bus with maximum real power \(|P|\) acts as the slack bus
required to account for line losses and serve as a generator, injecting real power to the system
Need for Slack Bus? or Why Generator Bus is chosen as Slack Bus?
Basically PS has only two buses - Load & Generator
In these buses power injected by generators and power drawn by loads are specified
But power loss in TL are not accounted
\[\begin{aligned} \left(\begin{array}{c} \text{Total (complex) power}\\ \text{loss in TL} \end{array}\right) & =\left(\begin{array}{c} \text{Sum of complex}\\ \text{power of generators} \end{array}\right)- \left(\begin{array}{c} \text{Sum of complex}\\ \text{power of loads} \end{array}\right) \end{aligned}\]
Losses can be estimated only if \(P\) and \(Q\) of all buses are known
Powers will be known only after load flow solutions are obtained
Since the loss of a line depends on the line current which, in turn, depends on the magnitudes and angles of voltages of the two buses connected to the line, it is rather difficult to estimate the loss without calculating the voltages and angles.
For this reason a generator bus is usually chosen as the slack bus without specifying its real power.
It is assumed that the generator connected to this bus will supply the balance of the real power required and the line losses.