Peak-ratio analysis method for enhancement of LOM protection using M class PMUs

A novel technique for loss of mains (LOM) detection, using Phasor Measurement Unit (PMU) data, is described in this paper. The technique, known as the Peak Ratio Analysis Method (PRAM), improves both sensitivity and stability of LOM protection when compared to prevailing techniques. The technique is based on a Rate of Change of Frequency (ROCOF) measurement from M-class PMUs, but the key novelty of the method lies in the fact that it employs a new “peak-ratio” analysis of the measured ROCOF waveform during any frequency disturbance to determine whether the potentially-islanded element of the network is grid connected or not. The proposed technique is described and several examples of its operation are compared with three competing LOM protection methods that have all been widely used by industry and/or reported in the literature: standard ROCOF, Phase Offset Relay (POR) and Phase Angle Difference (PAD) methods. It is shown that the PRAM technique exhibits comparable performance to the others, and in many cases improves upon their abilities, in particular for systems where the inertia of the main power system is reduced, which may arise in future systems with increased penetrations of renewable generation and HVDC infeed

A VSM (virtual synchronous machine) convertor control model suitable for RMS studies for resolving system operator/owner challenges

In recent years, it has become clear that reaching the targeted levels of renewable power generation poses problems, not only for basic infrastructure and generation/load balancing, but also in terms of fundamental network stability. In Ireland, the contribution from convertor-connected generation is already constrained to 50-55%, while recent studies of other networks suggest that any "penetration" of convertors above 65% could lead to instability. The phenomena have been observed both in RMS and high-fidelity EMT simulations of convertor-dominated power systems, and appears to be unavoidable when using the dq-axis current-source controllers within conventional grid-connected convertors. The high control bandwidth (>50 Hz) of these convertors also means that they cannot be effectively included within RMS type large-scale network models. The idea of "synthetic inertia" has been proposed in some publications as a mitigating solution but needs to be considered carefully, since if implemented incorrectly it has been shown to further destabilise the network at the critical small timescales and high frequencies. In this paper we present simple versions of a Virtual Synchronous Machine (VSM) model which is implemented and demonstrated in both transient and RMS based simulations. An important aspect of the VSM is that the controller’s bandwidth is low (<<50 Hz). This means that it can be modelled with reasonable accuracy in RMS simulation with time steps of the order of 2ms. From a system operator perspective, large-scale RMS simulations of entire countries or regions containing hundreds of VSM generators can be carried out with reasonable accuracy

Finding qualitative research: an evaluation of search strategies

BACKGROUND: Qualitative research makes an important contribution to our understanding of health and healthcare. However, qualitative evidence can be difficult to search for and identify, and the effectiveness of different types of search strategies is unknown. METHODS: Three search strategies for qualitative research in the example area of support for breast-feeding were evaluated using six electronic bibliographic databases. The strategies were based on using thesaurus terms, free-text terms and broad-based terms. These strategies were combined with recognised search terms for support for breast-feeding previously used in a Cochrane review. For each strategy, we evaluated the recall (potentially relevant records found) and precision (actually relevant records found). RESULTS: A total yield of 7420 potentially relevant records was retrieved by the three strategies combined. Of these, 262 were judged relevant. Using one strategy alone would miss relevant records. The broad-based strategy had the highest recall and the thesaurus strategy the highest precision. Precision was generally poor: 96% of records initially identified as potentially relevant were deemed irrelevant. Searching for qualitative research involves trade-offs between recall and precision. CONCLUSIONS: These findings confirm that strategies that attempt to maximise the number of potentially relevant records found are likely to result in a large number of false positives. The findings also suggest that a range of search terms is required to optimise searching for qualitative evidence. This underlines the problems of current methods for indexing qualitative research in bibliographic databases and indicates where improvements need to be made

Use of an inertia-less virtual synchronous machine within future power networks with high penetrations of converters

Conventional converter models for wind turbines and Voltage Source HVDC links, as submitted to System Operators, typically use dq-axis controllers with current injection (DQCI). Recent work carried out by the authors has proven that for DQCI converter-interfaced sources there are overall penetration limits, i.e. the 'tipping points' beyond which the system will become unstable. Initial investigations of this "tipping point", based on a reduced model of the transmission system of Great Britain using phasor simulation within DIgSILENT PowerFactory, are reviewed briefly in this paper. The 'tipping points' relating to maximum penetration of DQCI converter-interfaced sources are subsequently investigated in this paper using a higher fidelity three-phase dynamic power system model in Matlab Simulink. Additionally, a new converter controller, termed here as Virtual Synchronous Machine Zero Inertia (VSM0H), is described and implemented in the model. It is shown that, in principle, it is possible to significantly increase the penetration of converter based generation (up to 100% of installed capacity) without reaching a stability constraint

Effects of VSM convertor control on penetration limits of non-synchronous generation in the GB power system

2013 saw the presentation of a paper [1][2] to the wind integration workshop, which demonstrated 26 high convertor penetration scenarios, 17 of which introduced a type of instability in RMS models previously unseen by the researchers. It also provided an indication of the constraints necessary if NSG levels where to be limited, potentially placing practical limits on the amount of NSG which could be accommodated. It demonstrated that Synchronous Compensation (SC) could be used to mitigate these and other problems but this is believed to be an expensive solution. Further publications have demonstrated that convertor instability at high NSG extends beyond RMS models and is believed to occur in real systems [3]. In addition, Swing Equation Based Inertial Response (SEBIR) control, sometimes referred to as "Synthetic Inertia", has been shown to be ineffective as a countermeasure against the instability observed in [1][2] and can in some circumstances make it worse [4][5]. Whilst SEBIR improves RoCoF, its inability to address the wider range of problems resulted in the need for more comprehensive solutions. Several authors have proposed converters using principles aligned with VSM and VSM0H concepts and controllers using these concepts exist within marine power networks. This paper returns to the studies presented in [1][2], which used a reduced 36 node GB model in PowerFactory (PF). However here, some of the convertors are replaced with VSM convertor models described in [6] to investigate the effects on Instantaneous Penetration Level (IPL) limit of NSG in terms of transient stability and steady-state stability. These and further results presented demonstrate the potential of VSM, in mitigating the effects of various challenges associated with high NSG, potentially allowing 100% penetration

Component release and mechanical properties of endodontic sealers following incorporation of antimicrobial agents

Root canal sealers with antimicrobial activity are highly beneficial; therefore, their antimicrobial properties could be improved by incorporation of antimicrobial agents. In the present study, the release of the quaternary ammonium compounds from endodontic sealers admixed with either benzalkonium chloride (BC) or cetylpyridinium chloride (CPC) at loadings of 2% wt was monitored. The effect of these additives on the compressive strengths and their release from the sealers was determined after 1 and 4 weeks. All of the materials studied were found to be capable of releasing antimicrobial additive in useful quantities. The release of CPC occurred to a statistically significant greater extent than BC for all materials. The addition of both BC and CPC generally decreased the compressive strength of all the endodontic sealers, with the exception of CPC in AH Plus, where the compressive strength was significantly increased. This suggests that, for these endodontic sealers, the antimicrobial additives alter the setting chemistry. AH Plus is an epoxy-based material cured with an amine, and in this case the increase in compressive strength with CPC is attributed to an enhanced cure reaction with this system. In all other cases, the additive inhibited the cure reaction to a greater or lesser extent

Do divorcing couples become happier by breaking up?

Divorce is a leap in the dark. The paper investigates whether people who split up actually become happier. Using the British Household Panel Survey, we can observe an individual's level of psychological well-being in the years before and after divorce. Our results show that divorcing couples reap psychological gains from the dissolution of their marriages. Men and women benefit equally. The paper also studies the effects of bereavement, of having dependant children and of remarriage. We measure well-being by using general health questionnaire and life satisfaction scores

CSO and CARMA Observations of L1157. II. Chemical Complexity in the Shocked Outflow

L1157, a molecular dark cloud with an embedded Class 0 protostar possessing a bipolar outflow, is an excellent source for studying shock chemistry, including grain-surface chemistry prior to shocks, and post-shock, gas-phase processing. The L1157-B1 and B2 positions experienced shocks at an estimated ~2000 and 4000 years ago, respectively. Prior to these shock events, temperatures were too low for most complex organic molecules to undergo thermal desorption. Thus, the shocks should have liberated these molecules from the ice grain-surfaces en masse, evidenced by prior observations of SiO and multiple grain mantle species commonly associated with shocks. Grain species, such as OCS, CH3OH, and HNCO, all peak at different positions relative to species that are preferably formed in higher velocity shocks or repeatedly-shocked material, such as SiO and HCN. Here, we present high spatial resolution (~3") maps of CH3OH, HNCO, HCN, and HCO+ in the southern portion of the outflow containing B1 and B2, as observed with CARMA. The HNCO maps are the first interferometric observations of this species in L1157. The maps show distinct differences in the chemistry within the various shocked regions in L1157B. This is further supported through constraints of the molecular abundances using the non-LTE code RADEX (Van der Tak et al. 2007). We find the east/west chemical differentiation in C2 may be explained by the contrast of the shock's interaction with either cold, pristine material or warm, previously-shocked gas, as seen in enhanced HCN abundances. In addition, the enhancement of the HNCO abundance toward the the older shock, B2, suggests the importance of high-temperature O-chemistry in shocked regions.Comment: Accepted for publication in the Astrophysical Journa

Instantaneous penetration level limits of non-synchronous devices in the British power system

The installed capacity of non-synchronous devices (NSD), including renewable energy generation and other converter-interfaced equipment such as energy storage, bi-directional transfer links, electric vehicles, etc., is expected to increase and contribute a large proportion of total generation capacity in future power systems. Concerns have been expressed relating to operability and stability of systems with high penetrations of NSD, since NSD are typically decoupled from the grid via power electronic devices and consequently reduce the “natural” inertia, short-circuit levels and damping effects which are inherently provided by synchronous machines. It is therefore crucial to ensure secure and stable operation of power systems with high penetrations of NSD. This paper will show and quantify the instantaneous penetration level (IPL) limits of NSD connected to a simple example power system in terms of steady-state stability beyond which the system can become unstable or unacceptable, defined as “unviable”. The NSD used in this example will be a conventional dq-axis current injection (DQCI) convertor model. The paper will introduce a set of criteria relating to locking signal in converter phase-locked loop, frequency, rate of change of frequency and voltage magnitude, which will be used to determine the system viability and the IPL limit. It will also be shown that there are several factors that can potentially affect the IPL limits. Frequency and voltage droop slopes and filter time-constant for DQCI converter are varied and it is shown how these settings influence the IPL limits. Finally, to provide additional insight into network viability under high penetrations of NSD, a visualisation method referred here as “network frequency perturbation” is introduced to investigate responses of individual generators to a change in network frequency