Search

Enhancing the efficiency and the reliability of the data center are the technical challenges for maintaining the quality of services for the end-users in the data center operation. The energy consumption models of the data center components are pivotal for ensuring the optimal design of the internal facilities and limiting the energy consumption of the data center. The reliability modeling of the data center is also important since the end-user’s satisfaction depends on the availability of the data center services. In this review, the state-of-the-art and the research gaps of data center energy consumption and reliability modeling are identified, which could be beneficial for future research on data center design, planning, and operation. The energy consumption models of the data center components in major load sections i.e., information technology (IT), internal power conditioning system (IPCS), and cooling load section are systematically reviewed and classified, which reveals the advantages and disadvantages of the models for different applications. Based on this analysis and related findings it is concluded that the availability of the model parameters and variables are more important than the accuracy, and the energy consumption models are often necessary for data center reliability studies. Additionally, the lack of research on the IPCS consumption modeling is identified, while the IPCS power losses could cause reliability issues and should be considered with importance for designing the data center. The absence of a review on data center reliability analysis is identified that leads this paper to review the data center reliability assessment aspects, which is needed for ensuring the adaptation of new technologies and equipment in the data center. The state-of-the-art of the reliability indices, reliability models, and methodologies are systematically reviewed in this paper for the first time, where the methodologies are divided into two groups i.e., analytical and simulation-based approaches. There is a lack of research on the data center cooling section reliability analysis and the data center components’ failure data, which are identified as research gaps. In addition, the dependency of different load sections for reliability analysis of the data center is also included that shows the service reliability of the data center is impacted by the IPCS and the cooling section.

This paper gives an overview of the components and failure modes that should be considered when studying the reliability of grid-size Battery Energy Storage System (BESS). Next to failures of the primary component, a reliability study should consider the failure of the protection, failure of the communication, and failure of the control system. After all the diagnosed failures, qualitative fault tree analysis (FTA) based on Boolean logic is introduced as a basis of further analysis. Storage installations have a number of applications, each of which requires a different look at the reliability. The following applications are discussed in the paper: improving supply reliability, compensation power flows, market participation and frequency control.

Due to the increasing penetration of wind power, grid codes require that the wind farms remain connected to the grid during voltage dips. The harmonic distortion might change during the short-term reduction in the voltage magnitude. Although the distortion only occurs during the short period of the voltage dip, high levels of harmonic might cause mal-operation of the protective devices in a wind power plant. The main negative influence that harmonics can have on wind power plant protection is that it can act unnecessarily. This work analysis the changes in the harmonic content in the voltages and currents during dips in a DFIG-based wind power plant. The results showed that the total harmonic distortion for the voltages in the collector system and at the terminals of the wind turbines increases during asymmetrical dips. The currents were affected for both symmetrical and asymmetrical dips. The third and seventh harmonics were the voltage components that most increased. For the currents, the increase was in the third harmonic and interharmonics.

Hyper-scale data centers are used to host cloud computing interfaces to support the increasing demand for storage and computational resources. For achieving specific service level agreements (SLA), this infrastructure demands highly available cloud computing systems. It is necessary to analyze the server failure incidents to determine the way of improving the reliability of the system since the computational interruption causes financial losses for the data center owners. Regarding the reliability analysis, it is important to characterize the time to failure and time to repair of the servers. In this paper, a publicly available data set from Google cloud-cluster data center will be analyzed to find the distribution function for the time to failure and the time to repair for the servers in a cloud based data centers.

This work compares the response of DFIG-based wind turbines under voltage dips caused by transformer-energizing and caused by an asymmetrical fault. Voltage dips with similar retained voltage and duration and the two distinct causes are applied to an equivalent model of a DFIG-based wind park. The dips are different in terms of harmonic content and unbalance during the recovery after the dip. Due to its slow recovery, the transformer-energizing dip results in longer fluctuation in torque, longer overshoot in DC-link voltage, and longer recovery to the pre-dip active power. Also, due to its harmonic content, the harmonic distortion in the rotor and stator current is higher for the transformer-energizing dip. As the impact is different between the transformer-energizing dip and the fault-caused dip, it is recommended to consider both events separately during low-voltage-ride-through studies.

This work assesses the low voltage ride-through requirements and the dynamic behavior of a mixed PMSG/ DFIG wind power plant. Previous works in the literature applied voltage dips seen at the Point of Common Coupling (PCC) of the wind power plant (WPP) directly, which means that the propagation from the transmission grid to the low-voltage level was not considered. This work aims to show the expected operation of a wind power plant when submitted to voltage dips caused by faults at the transmission level, i.e considering the propagation. Equivalent models of a transmission system and a wind power plant were used to generate five types of dips and to access the behavior of the WPP during these events. The results show the expected ranges during the voltage dips for the active power of the generators as well as for overvoltages and overcurrents of the converters.

Load growth and addition of renewable energy generation occur in a way that makes the power grid being operated closer to its physical limits. Increasing the flexibility of the electrical power system is an essential step in ensuring a continued high reliability of the electricity supply. Dynamic line rating (DLR) can be utilized to increase the reliability of the system by ordering curtailment only when needed. In this paper, a probabilistic approach is introduced for the operational overload protection based on the probability distribution of the actual line capacity. That distribution is obtained by considering measurement and prediction errors in weather parameters as well as other uncertainties. The results indicate that probabilistic DLR based protection would allow operational decision-making based on a fair balance between dependability and security. This is not possible using the classical overload protection system.

This paper presents harmonic voltage measurements for a part of a distribution network including low-voltage (400 V) and medium-voltage (10 kV). Measurements were performed during 2017 and 2018 in the north of Sweden. This paper shows correlations between different harmonic orders in low voltage and the correlation between low and medium-voltage harmonics. This information can be used for harmonic propagation studies in distribution networks. The results showed significant correlation between the 5th and 7th harmonics for low-voltage side and very high correlation between 5th harmonic order in low-voltage and medium-voltage. It is also observed that 5th harmonic has almost the same values for all transformers connected to the same 40 kV network.

Histotripsy is a focused ultrasound therapy for tissue ablation with bubble cloud activity. Persistent bubble clouds reduce the ablative capacity of histotripsy. Thus, there is a need for bubble-specific imaging to track bubble cloud dissolution between the application of histotripsy pulses. In this study, standard plane wave, pulse inversion, and chirp-coded excitation imaging were utilized to visualize bubble clouds at a 2-kHz frame rate. For all imaging schemes, a monotonic decrease in the bubble cloud grayscale was observed over a 45-ms period following the histotripsy insonation. The change in bubble cloud grayscale was dependent on the imaging scheme, with faster grayscale reduction for larger peak negative pressures of the imaging pulse. Bubble-specific sequences resulted in faster decreases in the bubble cloud grayscale compared with standard plane wave imaging. Overall, these results highlight high-frame rate imaging as a means to monitor and modulate histotripsy bubble cloud dissolution.

Low voltage ride through (LVRT) of wind turbines is an important grid integration issue and the subject of many studies. However, in many such studies, the voltage dip waveforms used to test the performance of LVRT methods are not the one that can occur at the terminal of a wind turbine in reality. This paper provides a critical review of existing works and summarizes the power quality knowledge needed to study LVRT. Characteristics of voltage dips at the terminals of a wind turbine generator (WTG) will be analyzed based on realistic wind farm topology and transformer winding configuration. The impact of collection system transformer winding configuration on low voltage ride through of DFIG is revealed for the first time. Also, the impact of phase angle jump (PAJ) is shown in simulation. The changes of PAJ and point on wave (POW) characteristics in propagation between point of common connection (PCC) and terminal are analyzed to inspire further works. These issues are important but widely neglected by current works.

In the context of modern information technology (IT) industry, cloud computing is gaining popularity for big data handling. Therefore, IT service providers like Google, Facebook and Amazon are expanding their technical resources by building data centers to improve the data processing and data storage facilities under cloud service pattern. However, data centers consume a large amount of electrical energy. In recent years, a lot of research has been done to reduce the electrical energy consumption of data centers by high performance computing. However, very few researchers have focused on the electrical energy consumption by the electrical components inside the data center. In this paper, a component based electrical energy consumption modelling approach is presented to identify the losses of different components as well as their interactions to the total electrical energy consumption of the data center. The electrical energy consumption models of servers and other components are presented as a function of server utilization.

This paper introduces the terms aleatory and epistemic uncertainties for use in a stochastic hosting capacity method. The role these uncertainties play in the hosting capacity determination is illustrated. It is shown that distinction between aleatory (statistical) and epistemic (systematic) uncertainties is helpful to characterize the probability distributions correctly. For epistemic uncertainties, it is often challenging to obtain information on the probability distribution function. For aleatory uncertainties, a method for characterizing the probability distribution is presented. Aleatory uncertainties' data measurements are used to obtain a distribution best-fit. The background voltage measurement for a customer in a low-voltage distribution network is used to illustrate the method. Values were obtained for the distribution functions of the three-phase voltages. The used distribution functions are found to influence the resulting hosting capacity. This entails that there is need for measurements and data collection. A research challenge remaining concerns the stochastic model of epistemic uncertainty.

It is known for instance that voltage waveform distortion and network impedance have a significant impact on PV inverter current emissions. Because this, much research is still required to better understand their behavior and impact when multiple common household devices are placed to operate together in the same low-voltage installation. In particular, this paper addresses the harmonic impact of LED lamps on PV inverters performance considering different technologies and number of lamps. The analysis has been carried out with different scenarios considering two types of LED lamps, with and without power factor correction feature, and three different PV inverter technologies. The evaluation of the impacts is simply performed by frequency and time domain analysis, establishing the correlation between the devices current harmonics. The results obtained from the experiments have shown that LED lamps are prone to add a significant impact on the PV inverter current harmonics, and this impact is mainly dependent on the devices used technology.