Control of Electric Power Systems

A POWER SYSTEM is planned, designed and built to supply the consumers with electrical energy considering: • Economy • Quality • Supply security (Reliability) • Environmental impact The last point has become more and more important during the latest years. This can be seen from the fact that most of the larger companies nowadays have special departments responsible for that the companies follow laws and regulations within the environment area. The three first points above constitute the basis of the optimisation that the power companies must make regarding their investments and daily operation. It is evident that economical considerations must be regarded when deciding issues about quality and supply security for the system. The higher quality and supply security that is required, the more expensive the electrical energy becomes. Methods based on mathematical analysis and decision making theory (operations research) have been worked out to handle this optimisation problem, which contains several considerations/adjustments that are hard to formulate in a stringent mathematical form. As cheap and safe access to electrical energy is of great importance for almost all activities in a modern society, it is not surprising that the decisions makers in the power industry were pioneers to utilise advanced optimisation methods and other analysis tools for expansion and investment planning. In this compendium we will limit ourselves to discussing how a power system can be controlled to fulfill the constrains regarding quality. A very short introduction is also given about the overall control in a power systemwhich besides its aim to fulfill the quality even plays an important role in the supply security.

Three important factors that define power quality are:

• Frequency variations

• Voltage variations

• Waveform of voltage and current The two first factors will be dealt with in this course, but the last point is getting greater importance because more nonlinear components are connected to the electrical system, e.g. power electronics.

These can give rise to harmonics. It has been shown earlier that the frequency is a very good indicator on the active power balance in a power system. The frequency is constant when the same amount of electrical power is produced as consumed by the loads, including system losses. If this is not the case frequency changes will occur. It can also be noted that the frequency is the same in the whole system at steady state. This depends on that the active power easily can be transported in the system (that is why it has been possible to build the power systems of today’s size). It is often said that the active power is a global quantity, unlike the reactive power which is a local quantity, since reactive power cannot be transmitted over longer (electrical) distances. (This is because of that X normally is much greater than R in a power system, at least for transmission and sub-tra-transmission networks.) The frequency of the system is reduced when a load increase is not compensated for by a corresponding increase of the turbine power of the connected generators. The power deficit decelerates the generator rotors and consequently the frequency is reduced. Frequency reductions also arise when production is lost, e.g. as a consequence of failures in the system which lead to that protections disconnect the failed equipment. Too large reductions of the frequency could lead to system collapse, since a lot of equipment in the power stations, e.g. power supply systems, do not tolerate too low frequencies. A load reduction in the system which is not compensated for by a reduction of turbine power leads to a frequency increase. The permitted stationary frequency deviation in interconnected power systems is at normal operation typically 0.2 % or 0.1 Hz. There are also sometimes requirements on how large the difference between actual, physical time and the time corresponding to the integrated frequency from the system can be. This time difference is normally not permitted to be larger than 10s. It is also important that the voltage deviations in the system is limited. This is of importance for the connected loads, but a “good” voltage profile is also essential for keeping the losses low and for utilising the reactive reserves to establish a secure operation of the system. Voltage control is, as been pointed out earlier, a more local control than the frequency control. If the

voltage deviates from the set value in a node, the control action must be made in this or a nearby node