Hi Sajjad, the slack bus provides the net power required to balance the power flows in the network.

- what is the benefit of selecting slack bus? The slack bus allows the power flow equations to be solved (e.g. for modelling or control purposes).

- What would be the result of different slack bus selection in a system? This will change which bus (is modelled as) providing the net power required to balance the network.

- Is it possible to do power flow analysis without selecting a specific slack bus? In a way. You can actually set up a 'distributed slack bus' where multiple buses share the net power demand.

- how the power flow results are going to be validated considering a connection between two different country with different slack bus selection? Ultimately, validation comes from the actual power flows that are observed. Whether they match the power flow analysis will depend on how well the abstraction of a slack bus matches the actual network controls (as well as load forecast accuracy etc).

Hi Sajjad, the slack bus provides the net power required to balance the power flows in the network.

- what is the benefit of selecting slack bus? The slack bus allows the power flow equations to be solved (e.g. for modelling or control purposes).

- What would be the result of different slack bus selection in a system? This will change which bus (is modelled as) providing the net power required to balance the network.

- Is it possible to do power flow analysis without selecting a specific slack bus? In a way. You can actually set up a 'distributed slack bus' where multiple buses share the net power demand.

- how the power flow results are going to be validated considering a connection between two different country with different slack bus selection? Ultimately, validation comes from the actual power flows that are observed. Whether they match the power flow analysis will depend on how well the abstraction of a slack bus matches the actual network controls (as well as load forecast accuracy etc).

The slack bus is very important in finding the solutions to power flow problems in electric power systems engineering. It gives idea of the net power necessary for the balancing of power flows in the network. Identifying a slack bus facilitates the analysis and solution to the power flow problem. Analysis of power flows is possible without a slack bus, but having a slack bus much simplifies the entire process.

As you are very clear a load flow is the steady state operating condition of a power system for a particular loading condition. So we know the generating capability at various generating stations and loads at various load centres exactly. Losses vary depending on the flow through the lines and other equipments and this is not known to us. For accommodating this feature we keep the power floating in one generating unit and fix that as reference. Depending on the system slack bus has to be chosen. Basically an acceptable operating condition can be obtained by fixing a reference bus called as slack bus.

Slack bus – to balance the active and reactive power in the system. It is also known as the Reference Bus or the Swing Bus. The slack bus will serve as an angular reference for all other buses in the system, which is set to 0°. The voltage magnitude is also assumed to be 1 p.u. at the slack bus.

The slack bus provides or absorbs active and reactive power to and from the transmission line to provide for losses since these variables are unknown until the final solution is established. The slack bus is the only bus for which the system reference phase angle is defined. From this, the various angular differences can be calculated in the power flow equations. If a slack bus is not specified, then a generator bus with maximum real power |P| acts as the slack bus. A power system scheme can involve more than one slack bus.

Without selecting a slack bus we cannot proceed in load flow analysis. By using the load flow equations we can only calculate the difference of delta 1 and Delta to separate values of delta 1 and Delta 2 cannot be calculated. For this purpose 1 bus is selected as reference bus and the voltage at that bus is selected as reference.

Hi Sajjad, in real-world all the synchronous generating units of a power system may be considered "Slack Buses" since every time the power system is subjected to an increase/decrease of load, all units contribute according to its size and governor control mode to achieve the balance between Load and Generation.

In load flow simulations, the existence of a "Slack Bus" or "Reference" is needed to solve the load-flow mathematical model. If no Slack or Reference bus is considered, the resulting Jacobian matrix will be singular so the solution algorithm will fail in finding a solution.

If you choose different buses as Slack in different simulations it's highly probable that the system losses rendered by the simulations will be slightly different.

Jorge Calderon Khaled Eltag Rajagopalan Raghavan Hachimenum Nyebuchi Amadi Thomas Morstyn

Dear all, Thank you very much for your explanations. I would like to ask you for some references in this regard, and one more question, are the same statements you said valid for state estimation problem or not? When solving problem in distributed manner, is selecting slack bus in every area valid?

Hi Sajjad,

A distributed slack bus method is descibed in F. D. Galiana, A. J. Conejo, and I. Kockar, “Incremental transmission loss allocation under pool dispatch,” IEEE Trans. Power Syst., vol. 17, pp. 26–33, Feb. 2002.

For power flow, the bus selection should match the way the network is controlled. If there is a single generator which will provide any demand mismatch, this should be the slack bus. If instead, demand mismatch is shared between multiple generators, these should be used as distributed slack buses.

Will leave the question on state estimation to others with more experience.

Slack bus - it's the bus with EMF source that has constant magnitude and phase angle. It's similar like EMF in a casual electrical scheme with resistors. The phase angle of slack bus is the reference angle for other generators.

P,V=const bus -it's nonlinear EMF source, that has magnitude V and unknown phase angle phi, but we know that V*I*cos(phi)=P. That means EMF is nonlinear, and depends on current: E=E(I).

P,Q=const bus - it's simple nonliear load, Z=Z(I), so that

S=|I|^2*Z=P+j*Q

The load flow problem - it's solving of nonlienar system of equations.

You can try to solve the grid

E(slack)---Zs1--*-----Zs2---E(P,V=const)

|

ZLoad(P,Q=const)

Try to use Given-Find operators in mathcad.

The result of load flow solution is the calculation of an steady-state electromechanical process.

Physical meaning of slack bus - it's some station (or couple of stations) that provides the frequency regulation (they tryes to make frequency equals to constant 50/60 Hz value).

P,V=const bus - it's the station that works with constant active power, i.e. constatant torque M, because P=M*2*pi*f

For load flow analysis, the slack bus is the reference bus where we usually set the voltage at 1.0 pu and the angle to zero. Without these assumptions, how the solution can be converged. You can assume this bus as an infinite bus with unlimited power flow.

Slack bus(or) Ref bus- is the bus which has highest operating voltage (with magnitude 1 and phase angle 0)

A slack bus sets a voltage angle (delta) reference for all other buses in the system.

The real and reactive power flow in a transmission line depends upon the voltage angle difference between the two buses and hence the actual voltage angle is insignificant.

For the same reason the delta of the slack bus can be assumed to be any value. But it is usually assumed as zero degrees just for the sake of convenience.