Load-current response to severe changes of voltage

University essay from KTH/Elektroteknisk teori och konstruktion

Author: Shuhan Ma; [2017]

Keywords: ;

Abstract: Electric loads, such as computers, lamps, refrigerators and heaters connected to low-voltage distributionsystems in homes and offices differ widely in how their current responds to changes in the applied voltage.These voltage changes could be a total collapse of the voltage, a weak or strong reduction of amplitude,or a sudden phase-shift.This thesis investigates the currents into some modern power-electronic interfaced loads, at sub-cycletimescales, in response to sudden changes of AC voltage magnitude. One reason for this interest was anearlier project about fault-location methods in which the necessary level of modeling of loads wasquestioned [1]. A related issue that is also investigated is the response of such loads when Wye-connectedin a three-phase system with a high-impedance neutral such as a broken conductor.Several related matters have been studied a lot in other work. For example, active and reactive powerconsumptions of loads can change in response to the variation of voltage magnitude and frequency.Studies of power system oscillations, and angle and voltage stability, typically make use of such models.At distribution level, the relation of power consumption of loads to the feeder voltage has been includedin studies of Conservation Voltage Reduction (CVR). In these examples, the variation of voltage is typicallysmall, and the result is a value that summarizes an entire cycle (active or reactive power) without givinginformation about the current’s faster changes. Studies of voltage dip tolerance have instead workedwith larger changes of voltage, and short and variable durations, but without the resulting current beingthe interest.This thesis starts with a literature survey about different types of modern loads with differentcharacteristics, especially electronic loads which have become a far greater proportion of the totaldistribution-system load over the past several decades. It is known that the number of the electronicdevices has increased significantly due to the boom in the consumer electronic market. The most commonexamples of these loads include: computers, monitors, TVs and DVD players. Based on a survey of loadbehaviors and models, the current response of these loads to the voltage dips or recoveries are modeledby circuit simulation. Measurements of the currents into some computer equipment were performedwith voltage dips and recoveries, and compared to the results from models. The voltage amplitude dropsto 25%, 50% or 75% of its original value, and recovers afterwards. The current response is studied, focusingmainly during a short time period up to a few AC cycles. A similar mixture of simulation and measurementis then used to study the situation of the potential that arises at the neutral point of a three-phaseconnected electronic load when the neutral conductor has a high resistance.

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