Voltage source converters for mitigation of flicker caused by arc furnaces

Abstract: The thesis deals with flicker and how to mitigate it bymeans of a Voltage Source Converter (VSC). Flicker, orvariations in illumination intensity, is above a certain levelvery annoying. The background to flicker is voltage amplitudefluctuating in certain frequency intervals caused bytime-fluctuating loads. The thesis focuses on the electric arcfurnace which has been identified as one of the majorcontributors to flicker.State of the art- technology for flicker mitigation is theStatic Var Compensator (SVC) used for reactive power control.Also, a saturable reactor series connected with an arc furnacecuts load current peaks and thereby reduces voltage variationscausing flicker. Investigations have however shown that theperformance of this technology is in some cases not enough tomitigate the flicker to acceptable levels.A quasi steady-state approach has been used to find thatvoltage variations in the grid are due to both reactive andactive power variations of the load. Implementing apparatus forflicker mitigation, also the reactance of a transformer betweenthe connection point and the flicker sensitive point in thegrid has to be considered. It was shown that reactive powerovercompensation can be used to counter balante the voltagevariations caused by the active power. It was also shown thatvoltage components contributing to flicker are limited tosidebands of the fundamental voltage.Studing the voltage variations using instantaneous values ofcurrents and voltages created a new insight in the flickerprocess. Except for the instantaneous real and imaginarycurrents, also variations in the lime derivative of the realload current give rise to voltage amplitude modulation causingflicker.A suitable technology for flicker mitigation is the VSCoperated with Pulse Width Modulation (PWM). By using aswitching frequency in the order of 1- 2 kHz, an efficientmeans for flicker mitigation is obtained. Digital simulationswere performed with a VSC used for flicker mitigation in amodel of an art fumace installation to verify the flickermitigation algorithms. Investigations on measurement delay,switching frequency and compensator bandwidth were performedwith respect to flicker level. Studies in a real-time analoglow power model including an arc fumace and a VSC indicatedthat the earlier results were reliable.Keywords:flicker, mitigation, volmge source converter,pulse width modulation, PWM, am furnace, control, powerelectronics, analog model, saturable reactor, static varcompensator, SVC