Towards Developing A Unified Index For Power Quality Evaluation In Distributed Generation Systems

Currently, renewable energy-based power generation is rapidly growing across the world as a response to technical, economic, and environmental developments, as well as political and social initiatives. However, the transition to a renewable-based power and fuels system is challenging.

Excessive penetration of distributed generation units with their electronic power interfaces into distribution systems has introduced different power quality problems into these systems. The problems usually occur when the penetration level exceeds the capacity of a system to host more distributed generation units, which is called hosting capacity in the literature. These problems depend on many factors such as type, location, connection, voltage level, interface, control strategy, and size of the units.

Hosting capacity corresponds to the evaluation of distribution networks’ readiness for increasing amounts of distributed generation units. The determination of hosting capacity enables the stakeholder to quantify the impact of distributed generation units on the performance of the power system by using a set of assessment indices. The selection of these indices depends on factors of interest such as imbalance, voltage profile, harmonic distortion levels, and other power quality measures.

This necessitates working on how to bring together the various indices in a unified global index in order to assure consistent trade-off between pros and cons of the practice of renewable energy resources. Power quality is the capability of the grid to compensate or reject the disturbances. Thus, evaluation of power quality performance can be a key enabler for such a unified index.

Many standard indices measure each of the power quality disturbances, but, to date, no single measure can indicate the overall power quality of a distribution system, particularly in the presence of distributed generation units. Despite that introducing one index to evaluate overall power quality performance of a system will inspire new facilities in planning, monitoring, and assessment of integrated hybrid smart grid-based renewable energy systems.

In this work, we propose a unified index for power quality assessment in different distributed generation systems using the analytic hierarchy process. The unified index is determined in terms of six power quality performance parameters, namely voltage total harmonic distortion, current total harmonic distortion, frequency deviation, voltage sags score, voltage flicker, and power factor at different interface buses of different grid-connected distributed generation systems. The selected indices represent some of the common power quality problems.

The analytic hierarchy process as a structured decision-making tool for organizing different attributes is employed to apply a prioritizing scheme to get the most feasible solution and provide a comprehensive framework for quantifying importance (weights) of the considered power quality criteria in different scenarios. The analytic hierarchy process has several advantages such as simplicity, adaptability, and transparency. However, it has also some disadvantages such as interdependency between objectives and alternatives, and the need of assigning weights for pairwise comparison on the basis of data availability or designer’s experience to judge which choice is to be preferred.

The idea behind this is to evaluate overall power quality performance of different hybrid smart grid-connected distributed generation systems, taking into account the type of interface buses and in turn their priorities. This may help to investigate impacts of the distributed generation units on the power quality performance of a system in a specific, measurable, and practical manner. Simply, a bus that has low power quality index can be identified easily, and therefore overall power quality of the system can be evaluated depending on the aggregation of the buses’ indices. Further, we investigate the impacts of reactive power compensation, harmonic mitigation, and faults on the proposed unified index.

The results show that the hybridization of renewable energy systems is an essential way for increasing utilization and enhancing power quality levels in the distribution systems. Also, based on the proposed unified index results, for the compensated systems using the harmonic filters, better values of the index are obtained in the different systems but with different degrees of effectiveness. However, under the fault conditions, values of the indices increased considerably, which indicate the severity of faults on power systems operation and the need to use advanced compensators to avoid such shortcomings.

Finally, if not properly assessed, distributed generation systems can cause different power quality problems to the distribution systems. Hence, it is necessary to evaluate these issues and move forward towards putting a global power quality index into practice to sustain power quality of smart grid-connected distribution systems within acceptable limits in the long-term, particularly in the planning of new distributed generation installations in modern power systems.

These points are described and discussed in the article entitled A Unified Index for Power Quality Evaluation in Distributed Generation Systems, recently published in the journal Energy. This work was conducted by Shady Abdel Aleem, from 15th of May Higher Institute of Engineering, Ghada Elbasuony and Ahmed Ibrahim from Cairo University, and Adel Sharaf from Sharaf Energy Systems.