Test: BUS Classification - Electrical Engineering (EE) MCQ

Concept:
The size of the jacobian matrix is given by:
S = 2PQ + PV
where, PQ = No. of load buses
PV = No. of generator buses
The generator bus consists of 1 slack bus.
Calculation:
PQ = 150
PV = 50 = 1 slack bus + 49 PV bus
Size of the jacobian matrix = 2(150) + 49
Size of the jacobian matrix = 349 × 349

Slack bus:

• In load flow studies, it is assumed that the losses in the transmission network can be supplied by one of the generators instead of shared by all the generators so that it is easy to calculate the loss.
• The generator which supplies the entire transmission line losses and also shares the demand is called the slack bus. The remaining generators are only sharing the demand.
• Power for slack bus = Total power going into the system - Total p going out of the system + transmission line losses.
• This difference is called slack, and hence the name 'Slack Bus'.
• When the system is having a greater number of buses the calculations can begin at one of the bus. The bus where the calculations are started is called as a Reference bus.

To make possible variations in real and reactive powers of the slack bus during the iterative process to be of a small percentage of its generating capacity, the bus connected to the largest generating station is normally selected as the slack bus. Therefore, slack bus has to be a generator bus.

A bus in a power system is a line at which the several components of the power system like generators, loads, and feeders, etc., are connected.
The buses in a power system are associated with four quantities, these quantities are the following:

• The magnitude of the voltage
• Phase angle
• Active power
• Reactive power

In the load flow studies, two variable are known, and the other two is to determined.
Depends on the quantity to be specified the buses are classified into three categories as follow:

The table shown below shows the types of buses and the associated known and unknown value.

Generation Bus  or Voltage Control Bus:

• This bus is also called the P-V bus.
• on this bus, the voltage magnitude corresponding to generate voltage and true or active power P corresponding to its rating are specified.
• Voltage magnitude is maintained constant at a specified value by injection of reactive power.
• The reactive power generation (Q) and phase angle (δ) of the voltage is to be computed.

Load Bus:

• This is also called the P-Q bus
• at this bus, the active and reactive power is injected into the network.
• The magnitude and phase angle of the voltage is to be computed.
• Here the active power P and reactive power Q are specified, and the load bus voltage can be permitted within a tolerable value, i.e., 5 %.
• The phase angle of the voltage, i.e.δ is not very important for the load.

Slack, Swing, or Reference Bus:

• Slack bus in a power system absorbs or emits active or reactive power from the power system.
• The slack bus does not carry any load.
• At this bus, the magnitude and phase angle of the voltage is specified.
• The phase angle of the voltage is usually set equal to zero.

Classification of Buses:
The power system is nothing but the interconnection of the various bus.
Each of these buses is associated with four electrical parameters namely voltage with magnitude and phase angle, active power, and reactive power.

Slack Bus:

• Slack Bus is also known as Swing or Reference Bus.
• One of the generator buses is considered as a slack bus.
• The slack bus does not exist in real rather it is assumed for the consideration of losses occurring during power transmission.
• Actually, there exist only two buses in the power system, Load Bus and Generator Bus for which active power is specified.
• Since active power delivered by Generator Bus and consumed by Load Bus differ
• This means that a power loss equal to the difference between Generator Bus P and Load Bus P is occurring.
• This loss can only be calculated after the solution of Load Flow.
• Therefore to supply power loss, an extra generator bus is considered for which bus magnitude and voltage are specified, and active power and reactive power are to be calculated.
• This active power of slack bus is the equivalent power loss occurring in different systems.
• Generally, the phase angle of the slack bus is taken for reference for the entire load flow solution. Therefore this bus is also called Reference Bus.​

Load Bus:

• For load bus, real power P and reactive power Q are known but the magnitude and phase angle of bus voltage is unknown.
• It is desired to find the bus voltage using load flow analysis. At load bus, voltage is allowed to vary within some specified limit.

Generator Bus:

• Generator Bus is also called a voltage-controlled bus.
• The generator is connected to this bus.
• Therefore the bus voltage corresponding to generation voltage and active power generation corresponding to generator rating is specified for this bus.

The quantities specified for different type of buses shown in the below table.

Load flow analysis:

Concept:
The bus whose voltage and frequency remain constant even after the variation in the load is known as the infinite bus.
The alternators operating in parallel in a power system are the example of the infinite bus. The on and off of any of the alternators will not affect the working of the power system.

• The capacity of a parallel operating system is enormous, their voltage and frequency remain constant even after the disturbance of the load.
• The connection and disconnection of any of the machines will not affect the magnitude and phase of voltage and frequency of an infinite bus.
•  In an infinite bus system, the voltage and frequency always remain constant.
• The synchronous impedance of the bus is low because of the parallel operations of the machine.

Busbar:

• A busbar is a strip or bar of Copper, Brass, or Aluminum that conducts electricity within a switchboard, distribution board, substation, battery bank, or other electrical apparatus.
• Its main purpose is to conduct electricity, not to function as a structural member.
• Normally bus-bar is rated in terms of current, voltage, frequency and short time current based on the application.

Temperature rise in the Busbars:

• During the short-circuiting, the bus bar should be able to withstand the thermal as well as mechanical stress.
• The heat dissipation through convection and radiation during this short duration is negligible
• All the heat is observed by the busbar itself.

• The most relevant information needed to specify P at PV buses is the solution of economic load dispatch.
• Economic load dispatch is a precursor to the load flow study (LFS), i.e. to perform the LFS the first step is to perform the economic load dispatch.
• The economic load dispatch means the real and reactive power of the generator varies within certain limits and fulfills the load demand with less fuel cost.
• The economic scheduling of the generators aims to guarantee at all times the optimum combination of the generator connected to the system to supply the load demand.
• The economic load dispatch problem involves two separate steps. These are the online load dispatch and unit commitment.
• The unit commitment selects that unit which will anticipate the load of the system over the required period at minimum cost.
• The online load dispatch distributes the load among the generating unit which is parallel to the system in such a manner as to reduce the total cost of supplying. It also fulfills the minute to the minute requirement of the system.

Note: Generator not always operates at rated power P because as per the demand by the load, P value changes. Base value can be different for different loads.

Consider the voltage phase angles at buses 2 and 3 be Q₂ and Q₃ since, all the three buses have the equal voltage magnitude. Which is 1 pu, so, it is a D.C. load flow. The injections at Bus 2 and 3 are respectively P₂ = 0.1 Pu
P₃ = -0.2 Pu
P₁ + P₂ + P₃ = 0
P₁ – P₂ – P₃ = -0.1 + 0.2 = 0.1 Pu
Now, the apparent power delivered to base is,

The real power delivered by slack bus
P = P₁ |S| = (0.1) (100 × 10⁶)
= 10 × 10⁶ watt = 10 MW

Classification of buses:
Load Buses:

• In these buses, no generators are connected and hence the generated real power PGi and reactive power QGi are taken as zero.
• The load drawn by these buses is defined by real power -PLi and reactive power -QLi in which the negative sign accommodates for the power flowing out of the bus.
• These buses are sometimes referred to as P-Q bus. The objective of the load flow is to find the bus voltage magnitude |Vi| and its angle δi.

Voltage Controlled Buses:

• These are the buses where generators or voltage-controlled equipments are connected. Therefore the power generation in such buses 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 an automatic voltage regulator, we can specify constant PGi and |Vi| for these buses.
• These buses are also referred to as P-V buses. The reactive power supplied by the generator QGi depends on the system configuration and cannot be specified in advance.

Slack or Swing Bus:

• This bus sets the angular reference for all the other buses. Since it is the angle difference between two voltage sources that dictates the real and reactive power flow between them, the particular angle of the slack bus is not important.
• It sets the reference against which angles of all the other bus voltages are measured. For this reason, the angle of this bus is usually chosen as 0°. Furthermore, it is assumed that the magnitude of the voltage of this bus is known.