Kaveh Rahimi

Effects of photovoltaic systems on power quality

Renewable Energy Resources (RER) are growing steadily and they are projected to supply all the electricity demand in the future. Currently, wind and solar energy resources have the highest rates of growth, and specifically in the recent years, solar energy has been number one in growth rate among all types of renewable resources. However, dealing with the solar energys intermittent nature is the main challenge of its utilization. Fluctuations of received solar irradiance can cause significant variations to the output of Photovoltaic (PV) systems. Those output variations can also affect voltage and current at the Point of Common Coupling (PCC) and consequently, power quality of the system. In this work, fluctuations of a PV system due to a cloud shadow are simulated and their effects on Total Harmonic Distortion (THD), and Individual Harmonic Distortion (IHD) during the period in which the cloud shadow passes over the PV system are studied. Simulations results show that decrease in received irradiance caused by the cloud shadow can significantly impact the current THD of the system. Moreover, the effect of the impedance between the utility grid and the PCC on voltage THD is assessed.

Performance of PV generation feedback controllers: Power factor versus Volt-VAR control strategies

The variable nature of photovoltaic (PV) generation can cause voltage fluctuations in power distribution systems. Feedback control can be used to minimize the voltage fluctuations. This paper presents the results obtained from comparing the control performance of two types of PV generation feedback control, namely Volt-VAR control and constant power factor control. A three minute PV generation transient is used to evaluate controller performance, where the transient data used originated from one second measurements taken on an actual PV generator. Using the three minute transient, a set of parametric studies are performed on both feedback control strategies. The performance of the control strategies are compared as to voltage control on the distribution feeder and also to the effect that the control may have on transmission system voltage. In considering transmission system voltage, the reactive power drawn from the substation during the transient is evaluated. Simulation results suggest that the choice of control to be implemented should be based on both transmission and distribution system operational concerns.

Investigating PV Generation Induced Voltage Volatility for Customers Sharing a Distribution Service Transformer

The number of grid-connected rooftop solar photovoltaic (PV) systems is expected to increase significantly in the next few years. Many studies have been conducted on analyzing transmission level voltage stability with high PV penetration, and recent efforts have also analyzed voltage stability at the medium- and low-voltage distribution levels. However, those studies have not considered detailed distribution secondary modeling extending from the primary feeder to the service transformer and all the way through the distribution secondary connections and service drops. This study investigates how variable rooftop solar PV generation impacts voltages at customers sharing a service transformer. Several different types of secondary system topologies are taken into account. A set of rules is presented that can be used to ascertain the level of voltage volatility expected as the solar PV generation varies.

Smart Grid Big Data: Automating Analysis of Distribution Systems

A computer cloud based architecture for the automated analysis of distribution systems is presented. Smart Grid Big Data solutions are delivered by combining Advanced Metering Infrastructure (AMI), Supervisory Control and Data Acquisition (SCADA), Outage Management System (OMS), and weather measurements with big models and measurement driven analysis of the big models. Both daily analysis for management and real-time analysis for operations are considered. Two types of system models, As-Designed and As-Is models, are used in the solution.

Integrated transmission & distribution system modeling and analysis: Need & advantages

The primary objective of this paper is to highlight the need for and benefits of studying the steady state and dynamic response of power systems using three phase integrated transmission and distribution (T&D) system models (hereafter referred to as hybrid models). Modeling and simulation requirements for building and analyzing hybrid models are also discussed. Finally, results from steady state and dynamic simulations of a large hybrid model are presented to demonstrate insights that can be obtained from hybrid models which cannot be obtained from the study of transmission and distribution systems separately.

Application of Distributed Series Reactors in voltage balancing

Voltage unbalance is one of the main concerns in power quality studies. In the work here three-phase transmission system analysis is used to simulate voltage unbalance. Distributed Series Reactors (DSRs) are employed to improve voltage unbalance due to asymmetry in line parameters, unbalanced loading, and other causes, such as transmission line sending end voltage unbalances. The main advantage of the proposed approach is its dynamic flexibility to respond to many simultaneous sources of unbalance. Several cases are investigated covering various sources of unbalance. Moreover, different load types are analyzed and it is observed that load voltage dependency is an important factor in the number of DSRs required to correct voltage unbalances. Simulation results demonstrate that DSRs are very effective in improving voltage unbalance due to many sources.

Quasi-Steady-State computation of voltage flicker with cloud motion simulator

Renewable Energy Resources (RER), especially wind and solar energy resources, have been growing steadily, and supplying all electric power needs from renewable resources appears more achievable than ever. Solar energy has had the greatest growth rate among renewable energy resources in recent years. The main challenge of employing Photovoltaic (PV) systems is the intermittent nature of solar energy. The irradiance variations are mainly caused by cloud movements and the associated shadows moving over solar arrays. Thus, when considering high PV penetration, simulation of cloud shadow movement and its effect on the power system is of interest. In this work a Cloud Motion Simulator (CMS) that incorporates Quasi-Steady-State (QSS) power flow analysis is introduced. Moreover, short-term flicker severity based on the IEEE 1453–2015 standard is computed by the CMS, and the use of this standard is discussed. The effect of CMS parameters — cloud speed, number of clouds, width of clouds, and time interval between clouds — on short-term flicker is also investigated.

Application of Distributed Series Reactors in relieving congestion costs

In the last decades, generation and demand have been growing steadily. Transferring the new generation to load centers requires the expansion of transmission infrastructure. However, building new transmission lines is a long drawn and challenging process. Therefore, optimal use of transmission lines and increasing their transfer capacity are of interest to system operators. Lines congestion can affect transfer capacity of transmission lines and impose congestion costs to transmission system. This paper introduces the application of Distributed Series Reactors (DSR) in relieving lines congestion and consequently, congestion costs in power market. In the work here, two sets of Security Constrained Optimal Power flow (SCOPF) simulations were performed for different load levels; with and without employing DSR modules. Simulation results show that DSR modules can effectively relieve congestion costs when they are employed on congested transmission lines.

Effect of communication Time-Delay attacks on the performance of Automatic Generation Control

Communication is the main player in smart grid and the future of power system has been tied to open communication infrastructures. Although communication in power system provides many advantages, it also introduces new challenges which cyber security is one of them. This work studies the effect of Time-Delay (TD) attacks on the performance of Automatic Generation Control (AGC) in a multi-area power system. In this study, a three-area power system is developed and then time delay attacks are simulated to evaluate the performance of the AGC scheme during the attacks. Simulation results show that TD attacks can effectively degrade the performance of the AGC scheme and in some cases even disable it. In addition, it is observed that an area with higher inertia and load damping coefficient has a better AGC performance during TD attacks.

Importance of detailed modeling of loads/PV systems connected to secondary of distribution transformers

Residential solar Photovoltaic (PV) installations are increasing at a very high pace in the United States. In 2017 there are approximately one million residential solar PV installations in the US. A significant share of these installations are downstream of distribution transformers and thus connected to the secondary. To precisely analyze voltage variations induced by PV systems into distribution systems, accurate models of load and PV systems connected to the secondary side of distribution transformers are required. In the work here we consider two secondary circuit modeling approaches, simple secondary and detailed secondary models. In simple secondary models all loads and all PV generators below a distribution transformer are modeled as an aggregate load and an aggregate PV generator. In the detailed secondary models all loads and PV systems below the distribution transformers are modeled individually and secondary conductors and service drops are also modeled. Using a cloud motion simulator it is observed that employing the simple secondary models can lead to inaccurate and conservative results. Moreover, the locations with the greatest voltage changes are different in the two modeling approaches. This paper highlights the importance of utilizing detailed secondary models over simple secondary models in analyzing PV generation.