Power quality enhancement at distribution level utilizing the unified power quality conditioner (UPQC)

Power electronics based equipments are rapidly emerging as key components in the present modern power distribution system. Power processing utilizing these devices offer vast advantages such as flexible control, cost reduction, overall size optimization, etc. On the other hand, operation of these devices gives rise to some of the serious power quality problems, such as, the reactive power requirement and generation of harmonics that pollutes the power distribution system. Moreover, modern equipments are becoming highly sensitive to the voltage supplied to them. Increasing the generation capacities of existing power stations is difficult and expensive due to environmental constraints. Hence, improving the quality of power can not only make the power distribution systems healthier and more efficient, but also results in reduced power losses, and thus saving in terms of costs.
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\nActive power filters are widely used to tackle some of the important power quality problems. Recent trends are geared towards the realization of multitasking devices which can tackle several power quality problems simultaneously. The unified power quality conditioner (UPQC) is one of the most versatile active power filters that can compensate significant power quality issues, such as, voltage harmonics, voltage sag, voltage swell, voltage unbalance, voltage flicker, current harmonics, load reactive power, current unbalance, and neutral current.
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\nA UPQC consists of two voltage source inverters connected back to back with each other sharing a common self- supporting DC link. One inverter is controlled as a variable voltage source in same manner as in the series active power filter (APF), and the other as a variable current source which is similar in operation as that of the shunt APF. The existing literature suggests the dependency on shunt inverter to compensate the load reactive power demand. Moreover, the series inverter is always utilized to overcome all the voltage related problems. The voltage sags, swells and the flickers are short duration power quality problems. Hence, this traditional approach of utilizing the UPQC to compensate the power qualite problems shows a significant drawback of under usage of the available series inverter. Therefore, the utilization factor of the series inverter is much lower than that of the shunt inverter.
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\nThe present doctoral work is based on the philosophy of optimal utilization of the available resources in a most effective and efficient way to improve the product efficiency and to reduce the overall cost. This work proposes a novel control concept, termed as power angle control (PAC), in which both the series and shunt inverters share the load reactive power in co-ordination with each other without affecting the basic UPQC compensation capabilities. This eventually results in a better utilization of the series inverter, reduction in the shunt inverter rating to some extent and ultimately in the reduction of the overall cost of UPQC.
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\nMoreover, this thesis also introduces a new functionality for UPQC in which, it is possible to extent the UPQC based three-phase three-wire system to a three-phase four-wire system.
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\nThe developed PAC concept is successfully validated through digital simulation as well as extensive experimental investigations. The experimental results show that for the given laboratory test conditions with a highly inductive load, a boost in power angle 8=28° between the source and the resultant load voltage can reduce the shunt inverter rating by 50%. The performance evaluation of PAC approach under voltage sag, voltage swell, distorted source voltage and non-linear load conditions is also carried out. Under distorted source voltage (source voltage THD=6.6%) and the non-linear load (load current THD=23.4%), the UPQC with proposed PAC concept, effectively compensates the harmonics in source voltage (load voltage THD=3.2%), load current (source current THD= 2.94%) and shares the load reactive power between the series and shunt inverters, simultaneously.
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\nThis work was partially conducted in collaboration with Hydro-Quebec, Montreal and Institut de recherche d'Hydro-Quebec (IREQ), Varennes, Canada.

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