Slack bus

A ‘Slack’ bus (or ‘swing’ bus) is defined as Vδ bus, that is used to balance the active |P| & reactive power |Q| in the system while performing Load flow studies in Electrical Power Systems. For Power Engineers, Load flow study is very important as it explains the power system conditions at various intervals during operation.

Slack Bus is used to provide system losses by emitting or absorbing active/reactive power to/from the system. While this definition of the load flow problem is appropriate for a deterministic solution, it has an inherent drawback when dealing with uncertain input variables: the slack bus must absorb all uncertainties arising from the system and thus, will have the widest nodal power possibility (probability) distributions in the system. If even moderate amounts of uncertainty are allowed in a large system, the resulting distributions will frequently contain values well beyond the generating margins of the slack generator.

Description

The most common formulation of the Load Flow problem requires all input variables (PQ at loads, PV at generators) to be specified as deterministic values. Each set of specified values corresponds to one system state, which depends on some set of system conditions. Thus, when the input conditions are uncertain, there is a need for numerous scenarios to be analyzed. A load flow approach that could directly incorporate uncertainty into the solution process has been long recognized as useful. The results from such analysis would be expected to give solutions over the range of the uncertainties, i.e., solutions that are sets of values or regions instead of single operating points.

Load buses are of 3 types and are classified as :

1. PQ buses - here, the real power |P| and reactive power |Q| are specified. It is also known as Load Bus.
2. PV buses - here, the real power |P| and the voltage magnitude |V| are specified. It is also known as Generator Bus.
3. 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 is selected to provide/absorb active and reactive power to/from the transmission line to provide losses since these are unknown until the final solution of the problem. The slack bus is the only bus at 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| will be chosen as the Slack bus. There can be more than one slack bus in a given scheme.

Formulation of Load flow problem

A classic load flow problem consists of calculation of voltage magnitude and its phase angle at the buses, and also the active and reactive line flows for the specified terminal or bus conditions. Associated with each bus of a power system, there are 4 set of variables -

1. magnitude of voltage i.e. |V|
2. phase angle i.e. |δ|
3. active or real power i.e. |P|
4. reactive power i.e. |Q|

Based on these values, a bus may be classified into the above-mentioned three categories as -

The solution to the load flow problem requires two main steps via 'mathematical formulation of the problem' and 'application of numerical technique to solve the problem'. In a load flow study, real and reactive powers (i.e. complex power) cannot be fixed at all the buses as the net complex power flow into the network is not known in advance, and the system power losses are unknown until the load flow study is complete. It is therefore necessary to have one bus (i.e. the slack bus) at which complex power is unspecified so that it supplies the difference in the total system load plus losses and the sum of the complex powers specified at the remaining buses. By the same reasoning the slack bus must be a generator bus. The complex power allocated to this bus is determined as part of the solution. In order for the variations in real and reactive powers of the slack bus to be a small percentage of its generating capacity during the iteration process, the bus connected to the largest generating station is normally selected as the slack bus.

Since load flow problems generate non-linear equations that computers cannot solve quickly, numerical methods are required. The following methods are some commonly used algorithms that solve these non-linear equations:

1. Gauss Iterative Method.
2. Fast Decoupled Load Flow Method.
3. Gauss-Seidel Method.
4. Newton-Raphson Method.

See also

• Power Flow Study
• Power Engineering

References

1. L.P. Singh, "Advanced Power System Analysis & Dynamics" by New Age International, ISBN 81-224-1732-9.
2. I.J. Nagrath & D.P Kothari, "Modern Power System Analysis" by Tata-McGraw Hill, ISBN 978-0-07-049489-3, ISBN 0-07- 049489-4

External links

• Load Flow Studies