Application and control of series active conditioners in electrical distribution systems

Abstract

In recent years, the size of electric power systems has greatly increased and the requirements to ensure their proper operation have become highly demanding. Traditionally, the concept of voltage quality (or power quality, loosely speaking) is referred to the average voltage level at the point of common coupling. In most cases, an irregular voltage value is caused by contingencies in the electrical system such as short-circuit faults, which lead to over-currents in distribution feeders that result in a sudden reduction of the voltage level (commonly known as \voltage sags"). End users connected to the grid may be a ected by voltage sags, which can eventually lead to production downtime and, in some cases, equipment terminal damage. A more recent concept of voltage quality is not only tied to the average voltage level but also to the voltage waveform. In this sense, voltage harmonics are the major cause of voltage quality deterioration and they are essentially generated by non-linear equipment such as diode or thyristor recti ers acting on the system impedances. Harmonics may produce pulsation torques in large electric motors, extra iron losses in electric rotating machines, and extra copper losses in the whole system. A Dynamic Voltage Restorer (DVR) is a power electronics device conceived to protect sensitive loads against voltage sags and swells. A DVR is connected in series with an electrical distribution line and, typically, it consists of a voltage-source converter (VSC), a DC capacitor, a coupling transformer, batteries, and an AC lter. When a voltage sag takes place, the DVR injects the required voltage in series with the feeding line and the load voltage remains unchanged. This will protect sensitive loads and would also improve the low-voltage ride-through capability of distributed generators connected downstream the DVR. The main advantage of DVRs is that only a portion of the power consumed by the load is supplied from the batteries. This means that batteries can be made much smaller than in a typical UPS and cost can be reduced. These reductions in the battery size and cost make DVRs very attractive for high-power applications where a UPS may be infeasible. A Series Active Power Filter (SeAPF) is another power electronics device aimed at eliminating voltage or current harmonic distortion and it shares the same basic topology with a DVR. For example, if the grid voltage is polluted with harmonics or is unbalanced, a SeAPF can inject a series-compensating voltage and the load would see an ideal voltage. A SeAPF can be used to improve the load-voltage quality using an appropriate controller and this ltering capability can be added seamlessly to a DVR. A DVR able to compensate voltage harmonics is commonly known as a Series Active Conditioner (SAC), although it is sometimes called versatile DVR or static series compensator (SSC). This thesis deals with the application and control of a SAC in an electrical distribution system. This device is devoted to protect a sensitive load against a wide variety of voltage disturbances such as voltage sags, swells, and imbalances, as well as voltage harmonics. A state of the art is presented and several control alternatives are studied in detail, with emphasis on harmonic-control algorithms. Power- ow limits and the DC-link voltage control of the VSC are also studied in detail. All the proposals in this thesis were implemented and tested on a prototype. Finally, conclusions and guidelines for further research are presented.