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With the proliferation of inverter based Distributed Energy Resource (DER) deployments, especially with advanced grid support functions (GSFs), the risk of unintended islands has remained a significant safety concern among the distribution system operators. IEEE 1547 standard requires DER to "detect an island, cease to energize the area electric power system (Area EPS) and trip within 2 second of the island formation." In addition, because of aggregate impact of high penetration of DERs on system stability, IEEE 1547-2018 revision mandated several GSFs, including abnormal voltage and frequency ride-through and voltage/frequency regulation support capabilities for all DERs. These grid support functions aim to maintain stability during grid-connected operation and island preventions aim to destabilize the network when the grid is not present. These apparently opposite objectives lead to the skepticism about island prevention effectiveness together with grid support from smart inverters. This paper investigates the apparent conflict in objectives and provides answer by revealing the unique relationships between frequency, voltage, active and reactive power in grid following inverter dominated islands and the fundamental differences in inverter’s control during grid-connected and islanded condition. It also explains why ride-through, volt-var, frequency-watt, and volt-watt functions are not expected to impact island detection performance by smart inverters.

Open phase events are not rare in distribution grid with independent phase protection. IEEE 1547–2018 prohibits DER from back feeding into open phase conditions. However, detection of medium voltage (MV) open phase can be challenging for the low voltage connected DER. Depending on the winding of the interconnection transformer, generation and loading level, etc., DER inverter terminal voltage may remain within the normal operating range. This paper investigates an actual open-phase event and evaluates the DER inverters' capability to detect such conditions. Two detection methods based on voltage and current negative sequence components are studied. It is found that DER inverters are able to detect the MV side open phase events in most conditions, especially when the active detection method is utilized, but there are some limitations.

The penetration of renewable energy resources, such as wind and solar, is continuously growing in power system grids and their interconnection in low short-circuit (LSC) systems (aka weak grids) is becoming more frequent. Converter interfaced resources have controls and dynamics that differ from traditional synchronous generators and require sufficient modeling for potential instabilities to be analyzed and prevented both in planning and operations environment. In this work, instability scenarios related to converter controls of wind turbine generators (WTGs) connected to LSC systems are analyzed, and the limitations of using positive sequence models in these scenarios are demonstrated. In addition, the influence of various converter control parameters on the WTG instability is investigated. The analysis is performed on a portion of the American Electric Power (AEP) network with several hypothetical renewable energy resources.

Transmission grid topology control (TC) is an emerging technology proposed in recent years to improve power system performance by using e xisting transmission lines more efficiently. Incorporated into the optimal power flow (OPF) with different objective functions, TC can be used for various purposes, such as operational cost reduction and corrective actions. As a leader of employing TC in the power industry, the Pennsylvania New Jersey Maryland (PJM) Interconnection has identified and published a list of switching solutions for the purpose of thermal limit violation and voltage control. Moreover, with significant penetration of wind and solar generations installed in the grid, the optimal system topology varies with time caused by the uncertainty of intermittent nature. TC has a promising future to track the optimal topology for each hour to reduce the operational cost and the line capacity expansion cost. However, these benefits are only possible if we can overcome the stability issues in operation, which is the focus of our research. A line switching action may introduce a large disturbance in the system and thus transient instability becomes a potential security concern. In previous literature, transient instability is observed in line switching actions in different systems. As a result, some beneficial switching plans are abandoned for the sake of security. Therefore, the key problem is how to enhance system stability to enable beneficial switching actions. To achieve that, a proper index that can be calculated offline and then used online to provide preventive stabilizing control guidelines is desired for TC applications. However, existing indices such as critical clearing time, transient stability index and transient energy function are unsuitable for preventive stabilizing control in TC applications. In addition, these indices are usually used for deterministic studies where the uncertainties in the grid are not considered. In our research work, a new quantitative index, critical switching flow (CSF), is proposed to assess transient stability in TC. CSF is the maximum real power flow allowed on a transmission line so that the system is stable when switching off the line. When a line flow is greater than CSF, the system has a risk of instability when switching off the line. The rigorous mathematical derivation of CSF is provided on a two-machine system and then extended to multi-machine systems. CSF establishes a direct connection between the line flow measurement of a switching target line and system transient stability in TC. Thus, CSF has a natural advantage of providing explicit control instructions in TC over the aforementioned indices. A preventive stabilizing redispatch scheme based on CSF is then proposed to enhance system transient stability in TC applications. First, the CSF values are calculated offline in the day-ahead unit commitment and scheduling stage using our probabilistic algorithm based on PSS/E with precise dynamic models from the industry. The proposed two-stage Monte Carlo simulation algorithm will consider the uncertainties of loads and generations and find the statistically precise CSF, which shows the boundary between stable and unstable switching actions based on the worst scenario. This offline learning process will generate sufficient knowledge for online TC applications. Then, the CSF based preventive stabilizing redispatch scheme will provide control guidelines to find a stable trajectory for an online TC application. Once a switching plan is found unstable in the online stability check, the proposed scheme is carried out first to decrease the line flow to a safe value below the CSF to ensure the stability of the subsequent switching action. The proposed algorithm and scheme are tested on the modified IEEE-118 bus system, which has large scale of wind power and solar power installed. In the numerical studies, the calculated CSF values are valid to avoid all unstable scenarios. Moreover, the proposed redispatch scheme is activated when there are sufficient resources to redispatch and decrease the flow on the switching target line to a value below the CSF. The proposed scheme is activated in 371 out of 800 unstable switching cases in total. All the 371 activated cases are stabilized using our proposed preventive stabilizing redispatch scheme. The proposed scheme will not be activated for the rest of unstable cases because of insufficient resources to redispatch. And details of system response in the proposed scheme are also shown for validation and illustration.

Integration of converter-control based renewable generators (CCBGs) into power systems introduces new dynamics and influences system stability. One particular concern is its impact on power system oscillations. Among different types of CCBGs, doubly-fed induction generators (DFIGs) have attracted the most attention. Reference [1]–[3] use the DFIG model with detailed control loops developed by General Electric (GE) and study its impact on power system small signal oscillations. It is observed that the DFIG reactive power/voltage control loop introduces an oscillation mode whose frequency is in the range of electromechanical oscillations (0.1 Hz to 2 Hz) [1]. DFIG's influence on the electromechanical oscillations is found to be highly dependent on parameters of this control loop [1, 2]. In [3], eigenvalue analysis is conducted for the Western Electricity Coordinating Council (WECC) system with projected high CCBG penetration conditions. An oscillation mode involving both wind turbine and synchronous generator (SG) state variables is observed. It is found that the wind turbine reactive power/voltage control variables have the highest participation in this mode. The observations in [1]–[3] indicate interactions between the DFIG reactive power/voltage control mode (referred to as VAR controller mode hereafter) and the SG electromechanical mode. However, the mechanism of the interaction and the conditions for strong interaction is not explored. Moreover, how this interaction affects the mode shapes of the two modes is not studied. Since the CCBG mechanical dynamics are separated from the grid by the converter interface, coupling between the CCBG converter control modes and the electromechanical modes becomes a critical way by which the CCBG dynamics can influence the electromechanical oscillations and it deserves through study. Based on the mode coupling theory, this paper reveals the interaction mechanism between the VAR controller mode and the SG electromechanical mode. Impacts of this interaction on power system oscillations are demonstrated and discussed. First, the VAR controller mode is investigated using damping torque analysis. Impact of operating condition change on its damping and frequency is shown analytically. The coupling between this mode and an electromechanical inter-area mode is then explored in a two-area test system. It is demonstrated that when frequency of the VAR controller mode approaches the inter-area mode, mutual participation increases and the influence of DFIG dynamics on the inter-area mode is increased. Additionally, it is shown that both positive and negative effect on the inter-area mode damping can be imposed depending on the VAR controller parameters. Moreover, the influence of mode coupling on mode shape of the two modes is investigated and it is demonstrated that the VAR controller mode, if becomes unstable, can couple with a well damped inter-area mode and cause unstable inter-area oscillations. In other words, in this situation, the unstable local voltage oscillation at the DFIG wind farm will propagate to other areas of the system and cause detrimental effect to the system. This study has practical merits. Challenge of wind generator voltage control on weak grids has been identified in the Electric Reliability Council of Texas (ERCOT) system [4]. Voltage and power oscillations close to a wind power plant were observed in ERCOT, following the outage of a nearby transmission line [4] or update of its controller settings [5]. Voltage controller is identified as the source of oscillations [4, 5]. The study in this paper offers insights into the VAR controller mode and demonstrates that not only local oscillations, but also inter-area oscillations can be caused by improper setting of the DFIG VAR controller or change in system operating conditions close to the DFIG plant. The key findings and conclusions of this study are summarized as follows: The DFIG VAR controller mode is sensitive to the system operating condition close to the wind generator. The voltage control scheme of DFIG is more prone to instability than the reactive power control scheme and the damping of the VAR controller mode decreases when the system becomes weaker at the DFIG interconnection point.When frequency of the VAR controller mode approaches an inter-area mode, the followings are observed: 1) mutual participation of the two modes increases; 2) the influence of DFIG dynamics on the inter-area mode damping increases; 3) mode shape of the VAR controller mode changes to involve inter-area oscillation of the SGs. When mode coupling is strong between the two modes, unstable inter-area oscillations can be caused by two mechanisms: 1) a stable VAR controller mode contributes negative damping to the inter-area mode 2) the VAR controller mode becomes unstable and couples with a sufficiently damped inter-area mode (change in VAR controller mode shape) and cause inter-area oscillations. Both situations can be severely detrimental to the system and should be carefully studied to avoid them.Proper settings of the VAR controller can also increase the inter-area mode damping. Results of this study can be generalized to other types of CCBGs since they have the same VAR controller structure (based on models in PSS/E). Thus the potential coupling between the VAR controller mode and the inter-area mode is generic for CCBGs. [1] G. Tsourakis, B. Nomikos, and C. Vournas, “Effect of wind parks with doubly fed asynchronous generators on small-signal stability,” in Electr. Power Syst. Res., vol. 79, no. 1, pp. 190–200, 2009. [2] L. Fan, Z. Miao, and D. Osborn, “Impact of doubly fed wind turbine generation on inter-area oscillation damping,” in Proc. IEEE Power Eng. Soc. Gen. Meeting, 2008, pp. 1–8. [3] J. Quintero, V. Vittal, G. T. Heydt and H. Zhang, «The Impact of Increased Penetration of Converter Control-Based Generators on Power System Modes of Oscillation,» in IEEE Transactions on Power Systems, vol. 29, no. 5, pp. 2248-2256, Sept. 2014. [4] S-H. Huang et al. “Voltage Control Challenges on Weak Grids with High Penetration of Wind Generation: ERCOT Experience,” in Proc. IEEE PES General Meeting, San Diego, CA, July 2012, pp. 1–7. [5] Technology solutions for wind integration in ERCOT, [Online]. www.electrictechnologycenter.com/pdf/TPR2Rev0Chg0043014.pdf

Cyber security is a crucial factor for modern power system as many applications are heavily relied on the result of state estimation. Therefore, it is necessary to assess and enhance cyber security for new applications in power system. As an emerging technology, smart grid topology control has been investigated in stability and reliability perspectives while the associated cyber security issue is not studied before. In successful false data injection attack (FDIA) against AC state estimation, attacker could alter online stability check result by decreasing real power flow measurement on the switching target line to undermine physical system stability in topology control. The physical impact of FDIA on system control operation and stability are illustrated. The vulnerability is discussed on perfect FDIA and imperfect FDIA against residue based bad data detection and corresponding countermeasure is proposed to secure critical substations in the system. The vulnerability and countermeasure are demonstrated on IEEE 24 bus reliability test system (RTS).

This paper investigates the stability issues that might arise when incorporating transmission line switching into smart grid planning and operation. To demonstrate the feasibility of line switching, we use scenarios and dynamic simulations to demonstrate system security margin and online stability issues. Results show that proper line switching can resolve system emergency and boost system security margin, even though less transmission lines are used. Also it reveals that small signal instability can be triggered by line switching. It is necessary to apply sophisticated voltage controls and power system stabilizers (PSS) to avoid triggering rotor angle and voltage small signal instability for full benefits of line switching.

Accurate quantitative information of crack length and width is important for assessing the severity level of cracks and making accurate pavement maintenance decisions. Nevertheless, due to the noise and fluctuation at the crack edge of pavement image, itis difficult to precisely determine the edge correspondence of crack that is necessary for measuring the crack width at a specific pixel. Different automated or manually performed measurements are likely to yield different results at the same pixel. Instead of measuring the crack width on a pixel-by-pixel basis, this paper presents an accurate and robust segment-based method for measuring crack width. Firstly, based on the distinctive curved structure of pavement crack, a structured edge detector is trained to obtain the confidence map of crack edges. Secondly, with the crack edge map, the morphological operation is used to extract the crack skeleton which characterizes the propagation of the crack. Adaptive segmentation of the crack skeleton is performed to partition a crack curve into crack segments. Each crack segment has the same width because its edges become almost parallel after the segmentation. Finally, combining the structured edge confidence and grayscale contrast at crack edges, an enhanced edge map of crack is proposed to measure the width of each crack segment by translating the skeleton towards both edges. A large number of experiments taken on the synthetic and real-world pavement images demonstrate that the proposed method can accurately and robustly quantify various cracks with the average accuracy of 93.7% for crack width. It is promising for quantitative pavement condition assessment and maintenance.

The optical properties of materials can be characterized in terms of the complex refractive index. We developed a simple and efficient method for determining the complex refractive index of nanomaterials that uses the relationship between this parameter and the light spectrum measured for a specimen. The optical properties of nanoparticle and nanotube TiO2 films were investigated. The results show that the morphology of nano-TiO2 affects the complex refractive index spectrum and therefore influences light dispersion and absorption of the film. We also simulated the optical energy density distribution within equivalent plane films using complex refractive index data.

Stability needs to be guaranteed when incorporating active transmission topology control (TTC) into smart grid planning and operation. In this paper, the impacts of TTC on power system stability are analyzed with wind and hydraulic power penetration. Potential stability issues caused by TTC are demonstrated. Also, a stability assessment tool based on time-domain simulation is presented to efficiently assess transient, oscillatory (linear or nonlinear) and voltage stability of a power system following a proposed line switching operation.

Smart grid topology control makes power system more flexible and efficient. For example, it has been demonstrated that economic benefit and corrective action in contingency can be achieved by topology control. However, transient stability is a major concern in applying smart grid topology control since a potential large disturbance of line flows is involved. In this paper, a critical switching flow concept is proposed as a transient stability index to assess system transient stability associated with topology control. This new index is based on system worst scenario in offline study and thus can be used to evaluate switching plan and avoid transient instability in real time operation without online dynamic simulations. Monte Carlo based algorithm is developed to determine critical switching flow for planned system and validated in simplified WSCC 9 bus system and IEEE 24 bus reliability test system to demonstrate a potential use of the new concept.

Collusion tolerance is an essential requirement for digital fingerprinting, and existing collusion-resistant encoding and tracking methods are generally based on combinatorics. However, those methods have some deficiencies such as low tracking efficiency and large fingerprint database for matching. Owing to those weaknesses, a hybrid fingerprint encoding method based on residual characters is proposed in this paper. The fingerprint is generated by encoding the user information into a binary image composed of letter, numeral and Chinese character. As a result of the complex structure of Chinese character, more residue characters can be reserved when the fingerprint is damaged. Thus, the fingerprint has a good performance in anti-tampering. Meanwhile, it has the capacity for large users because of the diversity of Chinese character. In addition, the paper proposes a tracking method based on grid feature, which greatly reduces the computational complexity and improves the tracking efficiency. The robustness and collusion tolerance of the digital fingerprinting scheme are confirmed by theoretical analysis and experimental results.

In this paper, a semi-fragile watermark solution based on quantization index modulation in the wavelet region was proposed. The algorithm employs a compressed halftoned binary image as watermark and embeds it in the wavelet domain through quantization index modulation, which can not only identify the authenticity of the content of digital image, but also recover the tampered image approximately. Because the watermark is a low-resolution original image, which carries the original image information, so the original image is not required when authenticating, and the tampered region can be recovered according to the watermark. The method can tolerate a certain degree of compression distortion or noise. Beside, a secret key is used to select the embedded watermark bits and their embedded positions to increase the security of watermarking. Experimental results show this semi-fragile watermarking algorithm is effective and practical for content authentication.