Power system analysis software: 2nd generation programs; Graphics; Protection

2nd generation programs

The advent of the PC and modern computing equipment like iPads anc smartphones provide a universal platform on which most users and programs could come together. This process was further assisted when window-based programs reduced the need for such a high level of computer literacy on the part of users. Windows-based and Apple computers -- which has a similar capability to a PC but which is less popular with engineers -- and more powerful workstations, usually based on the Unix operating system, are used for software programs. Minicomputers and mainframe computers are also still in general use in universities and industry.

Hardware and software for power system operation and control required at utility control centers are usually sold as a total package. These are usually proprietary systems and the justification for a particular configuration requires input from many diverse groups within the utility.

Graphics

Two areas of improvement that stand out in this second wave of generally available programs are both associated with the graphical capabilities of computers. A good diagram can be more easily understood than many pages of text or tables.

The ability to produce graphical output of the results of an analysis has made the use of computers in all engineering fields, not just power system analysis, much easier.

Tabulated results are never easy to interpret. They are also often given to a greater degree of accuracy than the input data warrants. A graph of the results, where appropriate, can make the results very easy to interpret and if there is also an ability to graph any variable with other variables, especially if three dimensions can be utilized, then new and significant information can be quickly assimilated.

New packages became available for business and engineering which were based on either the spreadsheet or the database principle. These also had the ability to produce graphical output. It was no longer essential to know a programming language to do even quite complex engineering analysis. The programming was usually inefficient and obtaining results was more laborious, e.g. each iteration had to be started by hand. But, as engineers had to use these packages for other work, they became very convenient tools.

A word of caution: be careful that the results are graphed in an appropriate manner.

Most spreadsheet packages have very limited x-axis (horizontal) manipulation. Provided the x-axis data comes in regular steps, the results are acceptable. However, we have seen instances where very distorted graphs have been presented because of this problem.

Apart from the graphical interpretation of results, there are now several good packages that allow the analyst to enter the data graphically. It’s a great advantage to be able to develop a one-line diagram of a network directly with the computer. All the relevant system components can be included. Parameter data still require entry in a more orthodox manner but by merely clicking on a component, a data form for that component can be made available. The chances of omitting a component are greatly reduced with this type of data entry. Further, the same system diagram can be used to show the results of some analyses.

An extension of the network diagram input is to make the diagram relate to the actual topography. In these cases, the actual routes of transmission lines are shown and can be superimposed on computer-generated geographical maps. The lines in these cases have their lengths automatically established and, if the line characteristics are known, the line parameters can be calculated.

These topographical diagrams are an invaluable aid for power reticulation problems, for example, the minimum route length of reticulation given all the points of supply and the route constraints. Other optimization algorithms include determination of line sizes and switching operations.

The analysis techniques can be either linear or non-linear. If successful, the non-linear algorithm is more accurate but these techniques suffer from larger data storage requirements, greater computational time, and possible divergence. There are various possible optimization techniques that can and have been applied to this problem. There is no definitive answer and each type of problem may require a different choice.

The capability chart represents a method of graphically displaying power system performance. These charts are drawn on the complex power plane and define the real and reactive power that may be supplied from a point in the system during steady-state operation. The power available is depicted as a region on the plane and the boundaries of the region represent the critical operating limits of the system. The best known example of a capability chart is the operating chart of a synchronous machine. The power available from the generator is restricted by limiting values of the rotor current, stator current, turbine power (if a generator), and synchronous stability limits. Capability charts have been produced for transmission lines and HV DC converters.

Protection

The need to analyze protection schemes has resulted in the development of protection co ordination programs. Protection schemes can be divided into two major groupings: unit and non-unit schemes.

The first group contains schemes that protect a specific area of the system, i.e., a transformer, transmission line, generator or busbar. The most obvious example of unit protection schemes is based on Kerchief's current law - the sum of the currents entering an area of the system must be zero. Any deviation from this must indicate an abnormal current path. In these schemes, the effects of any disturbance or operating condition outside the area of interest are totally ignored and the protection must be designed to be stable above the maximum possible fault current that could flow through the protected area. Schemes can be made to extend across all sides of a transformer to account for the different currents at different voltage levels. Any analysis of these schemes is thus of more concern to the protection equipment manufacturers.

The non-unit schemes, while also intended to protect specific areas, have no fixed boundaries. As well as protecting their own designated areas, the protective zones can overlap into other areas. While this can be very beneficial for backup purposes, there can be a tendency for too great an area to be isolated if a fault is detected by different non unit schemes. The most simple of these schemes measures current and incorporates an inverse time characteristic into the protection operation to allow protection nearer to the fault to operate first. While this is relatively straightforward for radial schemes, in networks, where the current paths can be quite different depending on operating and maintenance strategies, protection can be difficult to set and optimum settings are probably impossible to achieve. It’s in these areas where protection software has become useful to manufacturers, consultants and utilities.

The very nature of protection schemes has changed from electromechanical devices, through electronic equivalents of the old devices, to highly sophisticated system analyzers (microprocessor-based relays or IEDs). They are computers in their own right and thus can be developed almost entirely by computer analysis techniques.