Introduction

Materials and Methods

Results

Discussion

Conclusions

References

Abstract

Background: Stroke mimic is a medical condition presenting with acute neurological deficit and simulate real stroke. The objective of this study was to evaluate the frequency and the various etiologies of stroke mimics in our center. Methods: We retrospectively reviewed the Thrombolysis Alert registry and we studied the frequency and characteristics of patients with stroke mimic. Results: Among 673 patients who were admitted to the emergency department within 4.5 hours for sudden focal neurological deficit suggestive of acute stroke, 105 patients (15.6 %) had a stroke mimic. The mean age of patients with mimics and brain strokes were 66.3 and 64.8, respectively. The mean Onset-to-door time was 136.82 minutes and the mean door-to-imaging time was 32.63 minutes in stroke mimics. Seizure (28.5%) was the most common diagnosis of stroke mimics followed by conversion disorder (25.7%). Conclusions: Stroke mimic is frequent and heterogeneous entity that can be difficult to identify. Fortunately, most previous studies show no harmful effects when using thrombolysis in a stroke mimic.

Introduction

Brain Stroke is a major cause of mortality and morbidity. In Morocco, the prevalence rate of stroke in all agesis estimated at 292 per 100,000. The management of ischemic stroke has seen major advances with intravenous thrombolysis (IVT) and endovascular thrombectomy (EVT). IVT has been the standard treatment for acute ischemic stroke (AIS) patients within 4.5 hours of symptom onset. Thrombolysis with tissue plasminogen activator is the first effective approved therapy for potentially disabling AIS. Recently there is more evidence suggesting that other thrombolytics such as tenecteplase may be even more effective. EVT is another effective therapy; it reduces the risk of death or dependency in patients with large vessel occlusion if applied within 6 hours. The recently published DAWN and DEFUSE 3 trials have shown that EVT is also beneficial 6-24 hours after the onset of symptoms in selected patients with favorable imaging brain. The diagnosis of acute ischemic stroke is usually easy by carrying out physical evaluation and studying images of the brain, however several disorders, manifested by an acute neurological deficit can mimic AIS. Stroke mimic term is applied in a clinical evaluation, describing those nonvascular conditions that simulate stroke, namely those presenting with sudden neurological deficit that resembles a stroke but is not related to a vascular cause. Stroke mimics are a major diagnostic challenge. The aim of this study is to evaluate the relative frequency and the various etiologies of stroke mimics in patients with clinical signs of stroke admitted in our emergency room for “Thrombolysis Alert.”

1- Introduction

2- Experimental setup

3- Cross wavelet transform

4- Wavelet and scales selection

5- Methodology

6- Results

7- Conclusions

References

Abstract

This paper presents a new wavelet analysis approach in partial discharges cable joint measurements in noisy environments. The proposed technique uses the Cross Wavelet Transform (XWT) to separate PD signals from noise and external disturbances in partial discharges measurements in cable joints using two opposite polarity sensors. The partial discharge measurements were performed during impulse and superimposed voltages, leading to a huge amount of noise and pulse shaped external disturbances. The XWT foundations, the experimental setup and the XWT methodology proposed are presented together with the results of the recognition of PD originated in the cable joint. In the experiments, 51,898 signals were acquired, in which 733 were PD signals from the joint and 51,165 corresponded to noise or external disturbances. The XWT performance was studied, finding that 97% of the PD signals were correctly separated by the technique proposed. The results demonstrate the effectivity of the XWT in separating PD signals from noise and external disturbances in this particular measuring system configuration.

Introduction

Nowadays, Partial Discharges (PD) detection is an essential tool for the diagnosis of high-voltage equipment because of their accuracy to detect and quantify defects and damages in the dielectric insulation, where the detection implies the measurement, acquisition, storage and processing of the PD phenomenon [1]. In general, the most widespread PD detection system is based on electrical measurements, in which the PD signals are acquired in the form of individual or series of electrical pulses. In offline PD cable field tests and in laboratory tests, capacitive coupled sensors installed at the cable ends are used [2,3]. In cable systems, statistically most of the partial discharges come from either the cable terminations or the cable joints, being necessary to locate them by time domain reflectometry techniques. In spite of the PD measurement has been exhaustively researched over the years, the separation of PD pulses from noise is one of the main challenges, especially in online applications. Therefore, noise contamination is one of the significant problems of PD detection [4], because noise, disturbances and interferences can give rise to complex Phase Resolved Partial Discharges (PRPD) patterns or clusters, leading to misleading interpretations [5]. For this reason, several studies [1,4,6–13] have focused on the PD pulses separation and denoising techniques for PD measurements. Among these studies, the wavelet transform has been broadly used because is capable of locating time and frequency components allowing the analysis of aperiodic signals with irregular and transition features, such as the partial discharges [1]. In the wavelet analysis techniques, the Discrete Wavelet Transform (DWT) has been used extensively for denoising PD signals. In general, in the DWT denoising, the wavelet coefficients are calculated for a given signal and then the coefficients are passed through a threshold (soft or hard) and followed by the reconstruction of the signal by taking the inverse wavelet transform of the modified DWT coefficients. However, a major problem that most of these denoising techniques face is the ingress of external interferences having time-frequency characteristics similar to the partial discharge signals (pulse shaped disturbances); for instance, periodic pulse shaped interferences from power electronics or another periodic switching [7], PD and corona discharges from the external power system, electrical pulses from switching operations, lightings, etc. This external noise can cause a false indication of PD activity, jeopardising the PD measurements as a diagnostic tool. To reduce the false indications, in PD measurement systems more than one electrical sensor (HFCTs, UHF antennas, coupling capacitors, etc.) is used, meaning that multiple waveforms are simultaneously acquired, because recording each signal through different sensors may provide extra useful information about the real nature of the waveform recorded. Tools like the correlation and trend analysis can provide the significance of relationships between the signals recorded [14].

1- Introduction

2- Set-up description

3- Verification of the PD measuring system

4- PD measurements

5- Improved PD measurements

6- Conclusions

References

Abstract

A partial discharge (PD) measuring system has been deployed in order to identify and measure PD in a high voltage (HV) cable joint under impulse and superimposed voltages under laboratory conditions. The challenge is to enable the detection of PD during the impulse conditions. The method of measurement has been investigated by introducing an artificial defect in the cable joint in a controlled way to create conditions for partial discharges to occur. Next the HV cable system is subjected to AC, impulse and superimposed voltage. Two high frequency current transformers (HFCT) installed at both ends of the cable joint were used to identify PD from the cable joint and to separate PD from disturbance. Transient voltage suppressors and spark gaps are applied to protect the measuring equipment. Band pass filters with selected characteristics are applied to suppress transient disturbances and increase the chance to detect PD during the impulse. PD signals are separated from transient disturbances during data post processing and by means of pulse polarity analysis. The developed system enables the detection of so-called main and reverse discharges respectively occurring during the rise and tail time of the superimposed impulse. The measurement results obtained show the effectiveness of the presented PD measuring system for investigating the effects of voltage transients on a HV cable system in laboratory conditions.

Introduction

Partial discharge measurements provide a useful tool to obtain information about discharging defects in high-voltage equipment. In power cables, PD occurs at insulation defects in particular in cable joints and terminations, especially at interfaces [1]. Therefore, PD measurement on cable systems can be considered a useful tool to diagnose insulation condition for both laboratory application and on-site application [2–4]. PD in power cables is normally measured under AC voltage by using the conventional technique defined by IEC 60270 [5]. In practice, power cables are not only subjected to AC operating voltage, but also to transient voltages such as lightning and switching impulses, which occasionally will be superimposed on the normal AC voltage. Those transient voltages will have an additional stress on the cable insulation. In that regard it is important to investigate PD under impulse and superimposed voltages. One of the challenges in measuring PD under impulse and superimposed voltages concerns the suppression of the disturbances caused by the transient voltages. In laboratory tests, the applied impulse voltage causes currents in the cable under test that disturb the PD measurement. So the PD measurement system needs to have a strong suppression of the disturbance. In such a case, the conventional PD technique is not suitable anymore. The unconventional method based on the measurements of electrical signals in MHz range is of more interest as a better alternative for these conditions [4,6–10]. Three circuits for PD detection under impulse are provided in [11] with a measurement frequency from hundreds of MHz to GHz, namely: the high frequency current transformer (HFCT) with multipole filter, the coupling capacitor with multipole filter, and the electromagnetic couplers. HFCTs or other sensors are commonly used with wide/ultrawide bandwidth together with filters and a digital oscilloscope to detect PD in insulation specimens or models under impulses [12–17]. A coupling capacitor was used to measure PD in material samples in cases where only impulse [18] and square wave voltage were applied [5]. Those PD measuring systems were able to detect PD during the impulse even during its front time. For superimposed impulses, PD was detected in laminated paper using a current transformer and a high-pass filter by Hayakawa et al. [19]. Nikjoo et al. [20] used a wideband detection system consisting of a coupling capacitor, a detection impedance and a low-pass filter to measure PD in oil-impregnated paper. However, in both works, PD was measured during AC cycles before and after impulses instead of during the impulses. Moreover, PD measurements in the above-mentioned works were performed on material specimens.

A wide-band, frequency-dependent five-terminal model is developed that represents one HVDC transformer unit in the French-English IFA2000 HVDC interconnection. Three such interconnected 1-ph units constitute one 3-ph transformer bank needed in 12-pulse conversion. The model is obtained via admittance frequency sweep measurements on the transformer’s terminals, including common-mode measurements to capture the high-impedance coupling to earth at lower frequencies. The data set is modified to reduce the magnetizing current to a realistic level by a novel eigenvalue scaling procedure. The final data set is subjected to modeling by a stable and passive rational model while utilizing a mode-revealing transformation to retain the accuracy of the small eigenvalues that are related to the high-impedance coupling to earth. The paper describes details related to the measurements and modeling steps as well as the many intermediate accuracy validations that were done. Also is described challenges in the measurements that resulted due to interference of the measurements by a nearby 400 kV overhead line. The model is applied in a time domain simulation of the complete HVDC link in normal operation where voltage waveforms resulting from line commutations are compared against those by a classical model with added stray capacitances.

Introduction

HVDC transformers are together with thyristor valves the fundamental components in any LCC-type HVDC converter. The protection of these transformers is therefore essential for the reliable operation of the HVDC link. Overvoltages due to switching events, lightning impulses and in-station flashovers on the AC side result in steep-fronted overvoltages that stress the transformer AC side insulation parts, and voltages are also transferred to the DC-side where they stress and interfere the thyristor components [1]. Conversely, line faults and lightning strokes to a DC overhead line may result in overvoltages that impose stresses on the transformer DC-side terminals. The protection of HVDC substation against overvoltages therefore requires insulation co-ordination studies to be performed to determine the appropriate location of surge arresters [1]. Insulation co-ordination studies requires the use of models with adequate accuracy up to several hundred kHz in order to represent the actual waveshapes of the overvoltages. The representation of the HVDC transformer is a main obstacle in such studies since frequency-dependent transformer models are in general not available. Therefore, simplified models are usually employed, often consisting of a 50 Hz model in combination with lumped capacitors.

1- Introduction

2- Theoretical Framework

3- Empirical Study

4- Results

5- Discussion and implications

References

Abstract

The expansion of sensors located within the shop floor area drastically expands the amount of information being available for planning procedures. However, it is questionable on how far ever more current data, leading to higher production planning frequencies, are able to contribute to a higher logistical performance of production planning and control (PPC). For this purpose, we conducted a study to confront frequently described potentials of real-time abilities with adverse effects consequent to scheduling nervousness. The results of a curvilinear regression indicate that the risks of scheduling nervousness can outweigh potential benefits due to immediate schedule adjustments.

Discussion and implications

Our study investigated the relation between scheduling frequencies applied in the course of production planning and the implications on performance measures of the production. The findings suggest that steadily increasing the update frequencies can be harmful to the actual performance concerning speed related measures of throughput time, delivery reliability as well as response capacity. Our study thus supports suggestions from several research approaches, recommending to put a more critical view on potential threats arising from frequent changes in the production plan [6, 15–18]. During the course of the empirical based study, the performance of manufacturing companies was assessed by measuring several speed related criteria, concerning the job order handling. By confirming hypothesis stated, the regression analysis emphasized an inverse curvilinear relation between the dependent variable (performance measure) and the independent variable quantifying the update frequency (model 3 in table 3). As another result of the regression analysis, the study furthermore stressed the importance regarding logistical objectives. The variable controlling for the main logistical objective showed a significant influence on the performance criterion.

1- Introduction

2- Experiment

3- Results and discussion

4- Conclusions

References

Abstract

In this paper, we present a detailed investigation of low frequency noise (LFN) for different n-MOSFET devices dedicated for memory applications. We investigate the impact of the gate oxide thickness (GOX) on LFN. We analyzed how the position, the existence and the composition of Lightly Doped Dopant (LDD) implant in the source/drain region affect the LFN performance of the device. The results demonstrates that the thinner gate oxide and the device without LDD improved the noise performance compared the devices with thick GOX and with LDD implants. On the other hand, the absence of LDD implant on one side of the MOSFET didn’t reveal a global trend for all measured devices. Finally, the different LDD implant composition resulted in different LFN performance which is gate area dependent. These results can be used from both process and design engineers to improve the LFN of n-MOSFET.

Introduction

Today, it is well known that LFN can be used as a characterization tool for the quality and the reliability of the devices [1-2]. It is obvious that the level of noise has a direct impact on the device quality. In addition, LFN can influence the design of an electronic circuit since it can limit the overall performance and operation. Thus, the noise investigation of special architecture devices is of paramount importance for device and design engineers. Single transistors designed for logic NVM of SONOS type memory applications [3] can be used for LFN comprehension analysis due to the special architecture they have. In this experiment five single cell n-MOSFET devices were designed for memory applications and used for noise analysis. In literature exists a number of publications referring to LFN characterization for logic NVM of SONOS type [4-7]. According to our knowledge, we could not find a prior work that used these type of devices for LFN improvement purposes and not for simple characterization. We tried not only to identify the main source of LFN in these devices but also to use this info for the improvement of LFN performance in CMOS technology. In MOSFETs, it is generally accepted that the flicker (1/f-like) noise originates either from carrier number fluctuations (CNF) (Eques 1, 2 Table 1) [1] or from Hooge mobility fluctuations [8]. The CNF noise is due to carrier exchange between the near-interface gate dielectric traps and the channel. The charge fluctuations in the gate dielectric could also induce fluctuations of the carrier mobility, giving rise to the so-called correlated mobility fluctuations (CMF) (Eques 3, 4 Table 1) [9-11].

1- Introduction

2- Experimental investigation

3- Seismic performance of frames

4- Analysis of test result

5- Conclusions and recommendations

References

Abstract

This paper presents an investigation on the seismic behavior of four reinforced concrete (RC) frames. The study is focused on the effect of the Aerated Lightweight Concrete (ALC) blocks infills on the seismic performance of the RC frames and the interaction between infills and surrounding frames. Four RC frames include a control specimen, frame with full-filled infills, frame with large window openings, and frame with eccentric door openings. Based on the low frequency cyclic loading experiments, hysteretic dissipation ability, stiffness degradation, characteristic displacement and load, failure pattern, flexural moments of columns, effective slab width, required ratio of column-to-beam strength, and column shear force are experimentally investigated and analyzed. Tests results indicate that the strength, the initial stiffness and the area of the hysteretic loop at the same load step were influenced significantly by infills when compared with the frame without masonry infills. This effect was reduced due to the large openings in the frame with infills. The column and beam mixed hinges failure was observed in the frame without infills while shear failure appeared at column ends in frames with infills. The testing results also show that inflection points were shifted to columns top because of the infills. Column shear forces were increased significantly due to the diagonal strut effect. Recommendations on useful effective slab width, the required ratio of column-to-beam strength and the equivalent strut width are made to take masonry infills into consideration in design.

Introduction

Masonry infills are frequently used in reinforced concrete (RC) structures. Since they are normally considered as architectural elements, the role of infills is underestimated in design code [1-4]. Seismic damage investigation indicated that RC frames were damaged showing “strong beam-weak column” failure, shear failure, and joint failure, which went against the original design intention. During strong earthquakes, the majority of RC frames collapsed or suffered a soft story failure due to few infills in the first floor (Fig. 1) [5]. Disproportionate openings led to short-column shear failure (Fig. 2) [6]. These damages illustrate that the infills have a significant effect on the whole structure and cannot be ignored. Thus, more researches should be carried out to understand the role of infills during strong earthquakes. During the last few decades, experimental studies on RC frames with masonry infills under static loads were carried out. Around 1960s, Polyakov [7] and Holmes [8] first found that brick infills could increase the strength and stiffness of frames, which brought infills effect into the sight of researchers. In order to compare seismic performance of RC frames with different infill materials and load routines, Bertero [9] carried out quasi-static cyclic load and monotonic load tests of 1/3 scaled 3-1/2 stories frames, Bertero [9] pointed out that the initial lateral stiffness, maximum lateral resistance, and effective viscous damping coefficient of frames increased significantly due to the effect of infills. Zovkic [10] also pointed out that the seismic performance was greatly influenced by the material of the infills. Armin [11] considered other influence factors such as strength of infills with respect to bounding frames, panel aspect ratio and vertical loads.

1- Introduction

2- System modelling

3- Proposed control framework for GISBs

4- Results and discussions

5- Conclusions

References

Abstract

Grid-interactive smart buildings with thermostatically-controlled loads can be modeled as virtual energy storage systems with dissipation, which have great potentials for providing grid ancillary services such as frequency support. In this paper, a new distributed aggregation control method is proposed for multiple grid-interactive smart buildings in one frequency control area (e.g. a residential community) to provide fast frequency support. The proposed method is based on the distributed sliding mode control via a leader-follower communication scheme. A leader control is designed to provide power and comfort/energy level references for the smart building aggregator based on the area frequency deviation, while references are tracked by each smart building using the proposed distributed sliding mode control. The stability of the proposed control method for grid-interactive smart buildings is proved by the Lyapunov method. With the proposed method, the external characteristics of the aggregated smart buildings will have good power tracking and energy recovery capability, which can effectively improve the system frequency response. In the aggregator, fair and efficient power and comfort/energy level sharing are achieved among all participating grid-interactive smart buildings. The proposed control scheme is tested on a three-area power system considering both system contingency and normal operation scenarios.

Background and motivation

In the last decade, the penetration of renewable energy resources into modern power systems is increasing rapidly. For example, in Singapore, the projected photovoltaics (PVs) generation will be increased to 350 MWp by 2020 [1]. However, high penetrations of renewable energy sources (RESs) such as solar PV and wind power have reduced power system inertia by replacing the synchronous machines [2], which can significantly challenge the system frequency regulation [3], control schemes [4] and transient stability [5]. In the meantime, fast and stochastic power variations of RESs will also lead to rapid frequency fluctuations [6]. It is well known that the power system frequency is an indicator of real-time power balancing between generation and consumption. So the frequency regulation becomes quite important in order to maintain a stable and efficient operation of the power systems. The severe frequency deviations can lead to load/generation trip-off, which jeopardize the stability and security of power systems as well as decline energy efficiency. A possible way to deal with the negative impact of renewable penetration is to increase the spinning reverses from power plants. However, involving more spinning reserves will bring additional fuel consumption, which is not an economic and energy efficient way to handle this problem.

I- Introduction

II- Ofets Fabrication Process

III- DC Characterization

IV- AC Characterization

V- Conclusions

References

Abstract

Thanks to recent progress in terms of materials properties, polymer field-effect transistors (FETs) operating in the MHz range can be achieved. However, further development toward challenging frequency ranges, for a field accustomed to slow electronic devices, has to be addressed with suitable device design and measurements methodologies. In this letter, we report n-type FETs based on a solution-processed polymer semiconductor where the critical features have been realized by a large-area compatible direct-writing technique, allowing to obtain a maximum frequency of transition of 19 MHz, as measured by means of ScatteringParameters (S-Parameters).This is the first report of solution-processed organic FETs characterized with SParameters.

INTRODUCTION

ORGANIC electronics underwent an impressive development and thanks to its peculiar properties, e.g. high degree of tunability of electronic properties and thin molecular films deposition with scalable processes at low temperature on a wide variety of substrates, represent one of the most promising candidates in the path to ubiquitous, flexible and lightweight low-cost electronics. To date, organic light emitting diodes (OLEDs) [1] have reached commercial maturity, and significant progress has been achieved for organic solar cells [2], photodetectors [3], biosensors [4] and circuits based on organic transistors [5]. In the case of field effect transistors (FETs), the use of polymer semiconductors has the advantage of achieving superior mechanical properties and easily enabling large-area fabrication techniques, such as printing. Excellent DC performances have been recently achieved owing to an improvement in the understanding of device physics [6], and steady increase in charge carrier mobility [7] and charge carrier injection [8]. Yet, progresses in DC performances are not usually matched by corresponding improvements in AC properties, since typical FET structures devised to test new polymer semiconductors are not optimized for frequency operation. It is instead highly demanded to enhance polymer FET maximum operational frequency, as this would enable a much broader application of this technology. For example, higher operational frequencies are crucial in drivers for high resolution flexible displays [9], radio frequency identification (RFID) systems for smart items identification [10] or wireless communication [11].

Generalized frequency division multiplexing (GFDM) is a block-based non-orthogonal multicarrier modulation scheme proposed for 5G PHY layer. In this paper, to efficiently coordinate the features of interference existing in the GFDM system, we propose a theoretical framework of the polar-coded GFDM (PC-GFDM) system, which allows jointly optimizing the combination of binary polar coding and GFDM modulation. The original GFDM channel is decomposed into multiple bit polarized channels by using a two-stage channel transform. The general modulation partition is performed in the first stage and the bit polarization transform is done in the second stage. Specifically, two schemes are considered for the first stage channel transform, multilevel coding (MLC) and bit-interleaved coded modulation (BICM). Based on the theorem we have proved in this paper that the capacities of channels corresponding to all transmitted symbols are identical, the MLC based PC-GFDM system is designed to optimize the combining of polar codes and GFDM system. Then the BICM based PC-GFDM system is designed to reduce the complexity and processing latency, which yields the suboptimal performance. Simulation results indicate that the proposed PC-GFDM systems significantly outperform the existing turbo-coded GFDM systems because of the joint design between the polar coding and the GFDM modulation.

Introduction

Polar codes proposed by Arıkan [1] are the first constructive codes that have been proved to achieve the symmetric capacity for the binary-input memoryless channels (BMCs). This capacity-achieving code family is based on a technique named channel polarization, which recently has been employed in many scenarios for signal processing. In 2013, Seidl et al. have introduced the channel polarization idea into the 2m-ary modulation scheme [2] and proposed two different polar-coded modulation (PCM) schemes, where the bit-interleaved coded modulation (BICM) [3] and multilevel coding (MLC) approaches are considered respectively. Later, by extending the channel polarization idea to non-orthogonal multiple access (NOMA) scenario, whereby the original NOMA channel is split into a set of binary-input channels under a three-stage channel transform, Dai et al design a polar-coded NOMA system [4] to jointly optimize the combination of binary polar coding, signal modulation and NOMA transmission. Similarly, polar-coded multiple-input multiple-output (MIMO) schemes [5] also have been developed to improve the system performance.

A periodic fixed-frequency staggered line leaky wave antenna (LWA) with wide-range beam scanning capacity is proposed in this paper. The reconfigurable antenna is based on a periodic staggered line LWA. The dispersion and Bloch characteristics of the periodic antenna is analyzed by the Macro Cell Method (MCM). The open-stop band (OSB) of the antenna can be suppressed effectively using the relationship between widths of oblique feeding lines and stubs. The reconfigurable characteristic is achieved by switches and microstrip lines between the edge gaps of each unit cell. A ‘‘supercell’’ is established by combining several reconfigurable unit cells, and the state is controlled by connecting or disconnecting the edge gaps of each unit cell using switches. Fixed-frequency beam-scanning capacity is implemented due to different propagation constants in different supercell states. The prototyped reconfigurable antenna can scan the beam between 130◦ and 54◦ at 4.5 GHz.

Introduction

The microstrip leaky wave antenna (MLWA) has attracted significant research attention since being developed in 1979 [1] due to unique advantages, such as simple structure, low profile, and beam-scanning capacity in the H-plane [2], [3]. MLWA designs with backward-to-forward beam-scanning capacity using the periodic waveguide structure have been reported [4]. Various periodic antenna designs have been developed, such as the periodic halfwidth MLWA [5], periodic triangle truncated DSPSL-based antenna [6], and a new type of circularly polarized printed periodic LWA structure [7]. However, the present of an open-stop band (OSB) limits the radiation of periodic structures in the broadside direction, causing a degraded radiation pattern and high return loss [8], [9]. Several methods have been proposed to suppress the OSB in periodic structures. The balanced condition has been confirmed as a useful condition in designing the CRLH LWA to reduce the OSB [10]. An OSB elimination technique, equal phase shift condition, for a class of periodic LWA with two types of uniform transmission lines in alternatively cascading arrangement is proposed in [11]. The beam scanning capacity of the MLWA depends on variations in frequency [12]. Most communication systems operate in the predefined frequency; thus, scanning at a selected frequency is highly desirable [13], [14]. One method is to manipulate the phase constant of the leaky micro-slotline using reactive loading across the slotline [15].

Abstract

1- Introduction

2- Convolutional neural networks for system modeling

3- Frequency domain analysis of random weights

4- CNN training with random weights

5-Simulations

6- Conclusion

References

Abstract

Randomized algorithms have been successfully applied in modelling dynamic system. How do random weights affect system identification and why do they sometimes work well? In this paper, we use the convolutional neural network (CNN) as an identification model to answer these questions. Since the convolution operation is an important property of the dynamic system and in the frequency domain it becomes the product, the CNN model is analyzed in the frequency domain. We first modify the CNN model, so that it can model both the input and the output series. Then we analyze the impact of the random weights of CNN in the frequency domain. We prove the existence of optimal weights and analyze the modeling accuracy under optimal weights and random weights. Through theoretical analysis, we propose a two-step training method and compare it with the random weight algorithm. The proposed CNN model with random weights is validated with three benchmark problems.

Introduction

To predict the future behavior of the dynamic system, to apply model-based control or to understand the physical process, system identification is needed. Neural networks are black box models, which only use input and output data. These data-based modeling methods still have many problems. The hyper-parameters must be defined before they are applied, for example, how many hidden layers and hidden nodes are needed [21]. The universal approximation theorem guarantees that the neural model can approach almost all continuous systems with enough hidden nodes, however, the fact of increasing the hidden nodes causes the over-fit problem [34]. The alternative method to increase the hidden nodes is to use more hidden layers. It is a basic idea of deep neural networks. Theory and application show that deep learning models achieve better training precision [2], and get impressive results on many difficult tasks [14]. The convolutional neural network (CNN) is one of the most important deep learning models [22][33]. The main difference between CNN and normal neural networks is the convolution operation. The first successful CNN is at early 90s for the classification of digits [23], which includes the invariances in two-dimensional forms using local connections and weight restrictions. The training uses the maximum likelihood estimation and the modified backpropagation algorithm [26]. To give a better understanding of how convolutional networks work, [39] proposed the de-convolutional technique to visualize the operations of the hidden layer. By using GPU (Graphic Processing Unit), faster learning and testing are obtained by [20]. It has been proven that CNNs have great performances in classification tasks, such as image processing [30]. CNNs are also applied for data regression and time series modeling. In [4], the time series is formed in the autoregressive model. In [12], CNN is applied to classify the time series.

I- Brief Introduction

II- Analysis of Single-Phase Short-Circuit Current at Converter Station

III- Analysis of Suppression Measures on Single-Phase Short-Circuit Current at Yibin and Fulong Converter Stations

IV- Conclusion and Advice

REFERENCES

Abstract

Aiming at the issue that the single-phase short-circuit current at the 500kV side of Yibin and Fulong converter stations has exceeded the breaking capacity of breaker, the reason is analyzed and the measures are put forward to reduce the single-phase short circuit, including limiting the output of thermal power near field, converter transformers neutral grounding by small reactance and 500kV lines adding series reactors. The limiting effects of single-phase short-circuit current and the impacts on power grid equipment operation and power supply reliability of different measures are compared. Feasible suggestions for reducing single-phase short-circuit current and improving power grid reliability are given.

BRIEF INTRODUCTION

Along with the expanding of the power grid construction and the strengthening of the AC and DC hybrid system, the short-circuit current level of the power grid is increasing year by year, and the single-phase short-circuit current of some buses is larger than the three-phase short-circuit current. Short-circuit current exceeding standard will bring difficulties to equipment selection, endanger the safety of people and equipment, which has become a main problem faced by power system planning and operation [1]. In the current Power System Analysis Software Package (PSASP), the converter transformer model is integrated in the DC system model. The DC system is treated as a load and does not provide short-circuit current when the model is used in the three-phase and single-phase short-circuit calculation. But in the actual power system, the converter transformer has a significant effect on the single-phase shortcircuit current. The capacity of converter transformers at Yibin and Fulong stations is respectively 9696MVA and 7706MVA, which is very large, and the converter stations have closely connection with neighboring 500kV power network [2]. Considering the influence of converter transformers on the zero sequence impedance and the centralized access of large capacity mating power supply such as Xiangjiaba and Xiluodu, the single-phase short-circuit current at the 500kV side of Yibin and Fulong converter stations has exceeded the three-phase short current or even exceeds the breaking capacity of breaker [3].