Mahyar Zarghami

A simple and effective approach for peak load shaving using Battery Storage Systems

This paper discusses a simple method to perform peak load shaving through the means of energy storage systems owned by a utility. Peak load shaving, also referred to as load leveling or peak shifting, consists of the schemes used to eliminate the peaks and valleys in the load profile. This practice offers direct and indirect benefits to utilities in generation costs, line loss reduction, and volt support. Prior work in peak load shaving has been mainly focused on optimization approaches implemented through methods such as non-linear and dynamic programming, or heuristic approaches such as particle-swarm optimization. The proposed algorithm for peak load shaving in this paper is based on a simple approach which compares the aggregated load profile with its average in a certain utilization period and shares the charge/discharge amongst energy storage devices based on the definition of energy bars and weighting factors. In particular, the paper focuses on the usage of Battery Energy Storage Systems (BESS) to accomplish this task. Results show that the proposed algorithm offers a simple, fast and effective way for peak-load shaving without heavy computational burdens often needed in other methods. As a result, it can be easily implemented in a utility main substation for controlling the charge/discharge of storage devices throughout the distribution system.

The Existence of Multiple Equilibria in the UPFC Power Injection Model

This letter shows the existence of multiple equilibria that arise from the use of the state model of the unified power flow controller (UPFC). These multiple equilibria can arise from a common power injection model for the same terminal conditions of shunt bus voltage and series active and reactive power injections. The multiple equilibria result in two or more sets of eigenvalues, some of which may indicate an unstable operating condition. Therefore, the use of the UPFC power injection model must be used with caution to ensure stable operation of the UPFC.

Investigating the impact of wake effect on wind farm aggregation

Aggregation methodologies for creating equivalent wind farm models are needed for power system transient stability studies involving large wind farms. One strong argument in the literature suggests single machine equivalent representation of wind farm with the assumption that all wind turbines receive the same incoming wind speed and thus operate at the same loading condition. In this paper, we examine how the validity of such single machine equivalent is affected under different wind speeds across a feeder in a wind farm with many WTGs. In particular, we compare the total active power output from the WTGs which indicates significant difference between single machine equivalent and full wind farm model under certain wind speed ranges and distances between the WTGs. We then conclude the need to use multi-machine equivalent representation for large wind farm in order to achieve adequate accuracy under the full range of wind conditions.

Review of Concepts to Increase Distributed Generation into the Distribution Network

Distributed Generation (DG) level of penetration is expected to be increasing due to the state and federal governments mandates for utilization of renewable resources. The current Distribution Network (DN) was not originally designed for integration of DG at high penetration levels. Improvements will need to be made to DN to facilitate safe and reliable interconnection of DG at higher penetration levels. This paper gathers and documents the major concerns such as, voltage, protection, and power quality related to DG interconnections as well as the issues with the existing design standards and criteria. Several different DN changes have been proposed to help resolve these issues. The focus of this paper is about the existing DN topology and DG integration issues, as well as documentation and discussion of the proposed solutions. Additional areas of research are identified for further consideration.

Optimal placement and signal selection for wide-area controlled UPFCs for damping power system oscillations

The paper discusses an optimal placement for UPFCs and an optimal method for the selection of global measurements in a wide-area controlled network for the purpose of damping power system oscillations. Both the placement and signal selection methods are optimized to damp interarea oscillations. Optimal UPFC placement is identified by comparing different candidate placements based on the total damping they produce. Optimal selection of output measurements is based on the projection of the right eigenvectors on outputs. After the selection of the desired output measurements, observer gains are designed by LMI approaches. Test results from the IEEE 57 bus test system indicate good potential in terms of selecting UPFC placements and output signals.

An estimative approach for CVR effectiveness using aggregated load modeling

In order to develop an estimate of the benefits of Conservation Voltage Reduction (CVR) in the distribution system, composite characteristics of system loads based on ZIP modeling has been extracted from measurements of electrical quantities at the main substation. This knowledge is further used in finding a quantitative estimation approach for predicting the effectiveness of substation transformer tap changes in reducing total power demand in different load conditions. The proposed algorithm is implemented on the IEEE 34 bus test system and the results are demonstrated. It is shown that the method can predict the effectiveness of CVR with acceptable accuracy.

Dynamic placement and signal selection for UPFCs in wide-area controlled power systems

This paper deals with the problem of damping inter-area oscillations in wide-area bulk power systems using unified power flow controllers. Dynamic placement and signal selection are two important issues when FACTS devices are deployed in the system. In the paper both problems have been investigated using a Most Dominant Branches table calculated based on modal analysis which shows the influence of active powers of the branches on inter-area modes of the system. First, dynamic placement has been explained in which the best placement candidates are selected based on their influence on inter-area modes. Next the paper deals with dynamic estimation of the states of the system based on selected global measurements. Estimation of the states is important in centralized control approaches since global feedback is not fully available. Simulations on the IEEE 118 bus test system show that the proposed approaches give valuable guidelines for dynamic placement and signal selection problems. Although the results of the discussed methods have been explored using UPFCs, applications can be extended to other series connected FACTS devices.

A state-space model for integration of battery energy storage systems in bulk power grids

Applications of Battery Energy Storage Systems (BESS) in the power grids will be expanded in the coming years due to factors such as decreasing costs of such systems. In a modern power network, BESS systems can be optimally controlled for peak-load shaving and other ancillary services such as volt-var control, system balancing and loss reduction. However, all these “system-level” controls will depend on the capability of each individual BESS to independently control the active and reactive power injection/absorption at its connected bus, and to maintain the voltage of the DC link between its AC/DC and DC/DC converters. As a result, understanding the behavior and control requirements for the operation of the BESS is necessary. In this paper, an active AC/DC rectifier and a bidirectional DC/DC converter have been modeled and analyzed for charging and discharging of the batteries in a BESS system. The models used in these analyses have been developed based on an integrated state-space equation set for the converters and the battery. This type of modeling is proper for demonstration of the effects of massively integrated storage systems in the bulk power grids.

Fault detection and isolation in a DC microgrid using a central processing unit

The goal of this paper is to design a protection system capable of detecting, isolating and locating high and low impedance ground faults in a DC multi-terminal system using a Digital Signal Processing (DSP) based control unit. Due to low current fluctuations during faults, it becomes difficult to detect high impedance faults using conventional methods. In order to locate faults, the DC ring bus will be divided into different zones and an algorithm will be developed to monitor currents in each zone. The proposed method has been tested through examples and is seen to be cost effective and easy to implement.

Complex power optimization of photovoltaic systems

Interconnection of Photovoltaic (PV) systems as Distributed Generation (DG) sources to the distribution circuits are rapidly increasing due to their benefits and low maintenance. However, massive penetration of these systems will result in issues which can impact the operation, controls, and protection of the grids. One of these issues is related to extra injection of power during daytime with high irradiation, which results in the backflow of power and system overvoltages. Current regulations do not allow photovoltaic inverters to participate in voltage or reactive power control. Hence, photovoltaic inverters are only allowed to inject active power to ensure maximum profits for investors. From the systems perspective, however, this type of operation may not result in the best practice. The goal of this paper is to show that coordinated active/reactive power control of the PV inverters can resolve the issues associated with voltage profile, while reducing the total demand of the system from the Utilitys perspective. For this purpose, a nonlinear optimization problem has been defined in which total demand of the system is minimized, considering system constraints such as voltage profile and line flows. Simulations on the IEEE 34 bus test system show that the proposed practice can significantly improve the system behavior.