Historical overview on dynamic reactive power compensation solutions from the begin of AC power transmission towards present applications

The transmission of electrical energy using AC started at the end of the 19th century and replaced smaller existing local DC distribution systems. By extending local supply areas and providing energy transfer over longer distances various problems regarding mainly voltage control and stability were observed caused mainly by reactive power unbalances in the systems. Switched reactive power compensation (shunt capacitors, shunt reactors) were primarily used to control the steady state system voltages. Dynamic reactive compensation were based on rotating machines, eg synchronous condensors. In the mid 60's of the 20^th century first static compensation devices, ie DC controlled reactors (mercury arc bulbs) and thyristor controlled devices (thyristor switched capacitors-TSC, thyristor controlled reactors-TCR) came up. Fast response times, lower losses and less maintenance requirements of thyristor controlled devices resolved the limitations of rotating machines and DC controlled devices. Evaluation of operating losses resulted more and more in the use of Static Var Compensators (SVC) which were built up by combinations of TCR and TSC branches. These shunt devices together with series connected devices TCSC (thyristor controlled series capacitors) provided a base for FACTS (Flexible AC transmission systems). FACTS allows transmission systems to be more efficiently used with regard to improved dynamic system voltage control on one side and higher power transfer capabilities on the other side. In AC transmission systems SVCs are installed presently with a total size of more than 100000 MVA. New power electronic devices (GTOs, IGCT, IGBT) were introduced to the FACTS market and allowed the use of current and voltage source converters (VSC) for providing fast reactive power compensation. With further improvement of controls, development of semiconductors and new arrangements of VSC technology to-days reactive power compensation is a key method to establish a reliable AC power transmission. The paper gives an overview from the early days of reactive power compensation towards the present transmission system situation. It compares early solutions with to-days devices, provides decisive factors for the steps in development of arrangements and discusses advantages of the present devices.