Power system analysis software: Finite element analysis; Grounding; Other programs; Further development of programs; Program suites; Conclusions

Finite element analysis

Finite element analysis is not normally used by power system engineers although it’s a common tool of high voltage and electrical machine engineers. It’s necessary, For example, where accurate machine representation is required. For example, in a unit connected HV DC terminal the generators are closely coupled to the rectifier bridges. The AC system at the rectifier end is isolated from all but its generator. There is no need for costly filters to reduce harmonics. Models of the synchronous machine suitable for a transient stability study can be obtained from actual machine tests. For fast transient analysis, a three-phase generator model can be used but it won’t account for harmonics. A finite element model of the generator provides the means of allowing real-time effects such as harmonics and saturation to be directly included. Any geometric irregularities in the generator can be accounted for and the studies can be done at the design stage rather than having to rely on measurements or extrapolation from manufactured machines to obtain circuit parameters. There is no reliance on estimated machine parameters. The disadvantages are the cost and time to run a simulation and it’s not suitable at present to integrate with existing transient stability programs, as it requires a high degree of expertise. As the finite element model is in this case used in a time simulation, part of the air gap is left unmeshed in the model. At each time step the rotor is placed in the desired position and the missing elements in the air gap region formed using the nodes on each side of the gap.

Grounding

The safe grounding of power system equipment is very important, especially as the short circuit capability of power systems continues to grow. Programs have been developed to evaluate and design grounding systems in areas containing major power equipment, such as substations and to evaluate the effects of fault current on remote, separately grounded equipment.

The connection to ground potential may consist of an ground mat of buried conductors, electrodes (earth rods), or both. The shape and dimensions of the electrodes, their locations, and the layout of an ground mat, plus the resistivity of the ground at different levels must be specified in order to evaluate the ground resistance. A grid of buried conductors and electrodes is usually considered to be all at the same potential. Where grid sections are joined by buried or aerial links, these links can have resistance allowing the grid sections to have different potentials. It’s usual to consider a buried link as capable of radiating current into the soil.

Various methods of representing the fault current are available. The current can be fixed or it can be determined from the short-circuit MVA and the busbar voltage. A more complex fault path may need to be constructed for faults remote from the site being analyzed.

From the analysis, the surface potential over the affected area can be evaluated and, from that, step and touch potentials calculated. Three-dimensional graphics of the surface potentials are very useful in highlighting problem areas.

Other software programs

There are too many other programs available to be discussed. For example, neither automatic generator control nor load forecasting has been included. However, an example of a small program that can stand alone or fit into other programs is given here.

In order to obtain the electrical parameters of overhead transmission lines and underground cables, utility programs have been developed. Transmission line parameter programs use the physical geometry of the conductors, the conductor type, and ground resistivity to calculate the electrical parameters of the line. Cable parameter programs use the physical dimensions of the cable, its construction, and its position in the ground. The results of these programs are usually fed directly to network analysis programs such as load flow or faults. The danger of errors introduced during transfer is thus minimized.

This is particularly true for three-phase analyses due to the volume of data involved.

Future of power-distribution programs

Recently there has been a shift in emphasis in the types of program being constructed.

Deregulation is making financial considerations a prime operating constraint. New programs are now being developed which assist in the buying and selling of energy through the electrical system.

Following on from the solution of the economic dispatch, 'time-of-use' pricing has been introduced into some power system operations. Under this system, the price of electricity at a given time reflects the marginal cost of generation at that time. As the marginal generator changes over time, so does the price of electricity.

The next stage is to price electricity not only on time but also on the place of use (nodal pricing). Thus, the cost of transportation of the energy from the producer to the user is included in the price. This can be a serious problem at present when power is exchanged between utilities. It will become increasingly common as the individual electricity producers and users set up contractual agreements for supply and use. A major problem at present is the lack of common agreement as to whether nodal pricing is the most appropriate mechanism for a deregulated wholesale electricity market. Clarification will occur as the structure of the industry changes.

Nodal pricing also takes into account other commercial and financial factors. These include the pricing of both generation and transmission constraints, the setting of a basis for transmission constraint hedges and for the economic dispatch of generation. The programs must be designed to give both the suppliers and consumers of energy the full opportunity costs of the operation of the power system.

Inherent in nodal pricing must be such factors as marginal cost pricing, short run price, and whether the price is ex ante (before) or ex post (after) the event. Thus far, the programming effort has concentrated on real power pricing but the cost of reactive power should also eventually be included.

The changes in the operation of power systems, which are occurring throughout the world at present, will inevitably force changes to many of the programs in use today and, as shown above, new programs will emerge.

Program suites

As more users become involved with a program, its quirks become less acceptable and it must become easy to use, i.e., user friendly. Second, with the availability of many different types of program, it became important to be able to transfer the results of one program to the input of another. If the user has access to the source code, this can often be done relatively quickly by generating an output file in a suitable format for the input of the second program. There has, therefore, been a great deal of attention devoted to creating common formats for data transfer as well as producing programs with easy data entry formats and good result processing capabilities.

Many good 'front end' programs are now available which allow the user to quickly write an analysis program and utilize the built-in IO features of the package. There are also several good general mathematical packages available. Much research work can now be done using tools such as these. The researcher is freed from the chore of developing algorithms and IO routines by using these standard packages. Not only that, extra software is being developed which can turn these general packages into specialist packages. It may well be that before long all software will be made to run on sophisticated developments of these types of package and the stand-alone program will fall into oblivion.

Conclusions

There are many more programs available than can be discussed here. Those that have been discussed are not necessarily more significant than those omitted. There are programs to help you with almost every power system problem you have and new software is constantly becoming available to solve the latest problems.

Make sure that programs you use are designed to do the job you require. Some programs make assumptions that give satisfactory results in most cases but may not be adequate for your particular case. No matter how sophisticated and friendly the program may appear, it’s the algorithm and processing of data that are the most important parts.

As programs become more complex and integrated, new errors (regressions) can be introduced. Wherever possible check the answers and always make sure they feel right.

Last, but not least, remember the golden rule of all computer programs and simulations: GIGO - Garbage In, Garbage Out. Make absolutely sure that all information is input into the program correctly, and that it’s defined in the right context and units. For example, a cable length defined as 300 miles instead of 300 m will make quite a substantial difference to the result of analyses.