Academic literature on the topic 'HVAC'

This paper discusses the use of the intelligent microcontroller and also discusses the results from the simulation application of fuzzy logic theory to the control of the high voltage direct and alternation current (HVDC)& (HVAC) power station systems. The application considered their implementation in both low and high level control systems in HVDC& HVAC power station systems. The results for the fuzzy logic based controller shows many improvements compared to the conventional HVDC& HVAC control system. The fuzzy logic based controller concept was further successfully extended to high level control of optimization problems such as the power swings. Based on simulation results, HVDC and HVAC breaker design are online protection against unwanted incidents happening to the system.

The concept of hybrid high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) grid systems brings a massive advantage to reduce AC line loading, increased utilization of network infrastructure, and lower operational costs. However, it comes with issues, such as integration challenges, control strategies, optimization control, and security. The combined objectives in hybrid HVAC–HVDC grids are to achieve the fast regulation of DC voltage and frequency, optimal power flow, and stable operation during normal and abnormal conditions. The rise in hybrid HVAC–HVDC grids and associated issues are reviewed in this study along with state-of-the-art literature and developments that focus on modeling robust droop control, load frequency control, and DC voltage regulation techniques. The definitions, characteristics, and classifications of key issues are introduced. The paper summaries the key insights of hybrid HVAC–HVDC grids, current developments, and future research directions and prospects, which have led to the evolution of this field. Therefore, the motivation, novelty, and the main contribution of the survey is to comprehensively analyze the integration challenges, implemented control algorithms, employed optimization algorithms, and major security challenges of hybrid HVAC–HVDC systems. Moreover, future research prospects are identified, such as security algorithms’ constraints, dynamic contingency modeling, and cost-effective and reliable operation.

Power transmission is a key link in power system. As the increase of power supply, the transmission capacity of the lines should be enlarged too. In the developed area, the right-of-way for transmission line is hard to be obtained. And converting the existing HVAC overhead transmission lines using HVDC technology could enhance the transmission capability. There are three possible plans for different HVAC transmission lines: single-pole HVDC, bi-pole HVDC and tri-pole HVDC.

High Voltage Direct Current Transmission (HVDC) is considered a better solution for bulk long distance transmissions. The increased use of HVDC is a result of its advantages over the HVAC systems and especially of its fault stability nature. A better solution is proposed by using a Voltage Source Controlled–HVDC as one of the infeed for the Multi-Infeed HVDC (MIDC or MI-HVDC) systems. The main advantage with the VSC converter is its flexible power control which enhances the stability of the MIDC systems. In this paper, the behavior of an HVDC system is compared with that of an HVAC during faults. A Hybrid HVDC system that includes a LCC as a rectifier unit and a VSC converter as the inverter is being proposed. It is considered suitable for MIDC systems and particularly for supplying a weak AC system. The performance of the system during steady state and transient conditions for all the proposed topologies including HVDC, HVAC and Hybrid HVDC are studied in MATLAB/SIMULINK. All of the proposed control strategies are evaluated via a series of simulation case studies.

The development of efficient and reliable offshore electrical transmission infrastructure is a key factor in the proliferation of offshore wind farms (OWFs). Traditionally, high-voltage AC (HVAC) transmission has been used for OWFs. Recently, voltage-source-converter-based (VSC-based) high-voltage DC (VSC-HVDC) transmission technologies have also been considered due to their grid-forming capabilities. Diode-rectifier-based (DR-based) HVDC (DR-HVDC) transmission is also getting attention due to its increased reliability and reduced offshore platform footprint. Parallel operation of transmission systems using such technologies can be expected in the near future as new OWFs are planned in the vicinity of existing ones, with connections to more than one onshore AC system. This work addresses the control and parallel operation of three transmission links: VSC-HVDC, DR-HVDC, and HVAC, connecting a large OWF (cluster) to three different onshore AC systems. The HVAC link forms the offshore AC grid, while the diode rectifier and the wind farm are synchronized to this grid voltage. The offshore HVDC converter can operate in grid-following or grid-forming mode, depending on the requirement. The contributions of this paper are threefold. (1) Novel DR- and VSC-HVDC control methods are proposed for the parallel operation of the three transmission systems. (2) An effective control method for the offshore converter of VSC-HVDC is proposed such that it can effectively operate as either a grid-following or a grid-forming converter. (3) A novel phase-locked loop (PLL) control for VSC-HVDC is proposed for the easy transition from the grid-following to the grid-forming converter in case the HVAC link trips. Dynamic simulations in PSCAD validate the ability of the proposed controllers to ride through faults and transition between grid-following and grid-forming operation.

Taiwan is developing the renewable energy actively, in which the wind energy is seen as one of important resources. However, the suitable locations for wind farm constructions are less and less on the shore, and the trend of wind farm development in the future will toward to offshore where the installation capacity of the wind farm could reach hundreds of megawatts. As the installation capacity of the wind farm increases, the effects on the interconnected AC grid are also more notable. In this paper, the off-peak system in Taiwan is used as a studied system in which the Penghu area and Taiwan grid is connected by submarine cables. This study explores the wind farm transmission system including HVAC, HVDC, and hybrid HVAC-HVDC systems and compares the differences of their impact on the system.

There are considerable power losses in Indonesia’s SUMBAGUT 150 kV transmission High Voltage Alternating Current Network (HVAC) system. These power losses and the voltage profile are critical problems in the transmission network system. This research provides one possible way to reduce power losses involving the use of a High Voltage Direct Current (HVDC) network system. Determining the location to convert HVAC into HVDC is very important. The authors of the current study used Particle Swarm Optimization (PSO) to determine the optimal location on the 150 kV SUMBAGUT HVAC transmission network system. The study results show that, before using the HVDC network system, the power loss was 68.41 MW. On the other hand, power loss with the conversion of one transmission line to HVDC was 57.31 MW for “Paya Pasir–Paya Geli” (efficiency 16.22%), 51.79 MW for “Paya Pasir–Sei Rotan” (efficiency 24.29%), and 60.8 MW for “Renun–Sisikalang” (efficiency 110.12%). The power loss with the conversion of two transmission lines to HVDC was 45.7 MW for “Paya Pasir–Paya Geli” and “Paya Pasir–Sei Rotan” (efficiency 33.19%), 44.95 MW for “Paya Pasir–Paya Geli” and “Renun–Sidikalang” (efficiency 26.98%), and 44.69 MW for “Paya Pasir–Sei Rotan” and “Renun–Sidikalang” (efficiency 34.67%). The power loss with the conversion of three transmission lines to HVDC was 38.71 MW for “Paya Pasir–Paya Geli,” “Paya Pasir–Sei Rotan,” and “Renun–Sidikalang” (efficiency 41.41%).

Large-scale offshore wind power has been developed in recent years, and two possibilities are considered for the transmission system between the offshore wind farms and the onshore grid: VSC-HVDC and HVAC. The paper aims to compare both systems for 300MW offshore wind farms, 25 km, 50km and 75km from the Point of Common Coupling on an economic basis using a DCF(discounted cash flow) analysis. A linear fit to the net present values has been made, obtaining the break-even distance of 47km. When the transmission distance is shorter than 47km, the HVAC system is more economic, otherwise the VSC-HVDC system is a more cost-efficient option.

Ion flow field is an important aspect of high voltage direct current (HVDC) transmission lines. In this paper, we apply the upwind finite volume method for solving the ion flow field of three HVDC configurations to clarify the effect of the wind and the role of shielding conductors. For the monopolar configuration installation, the ground current distribution with underbuilt shield wires was studied numerically and experimentally. For the ±250 kV bipolar configuration, the calculated peak electric field and current density are verified with the values in a reference. The ±500 kV bipolar configuration is used to investigate the change in electric field and ion current within the same corridors of the existing HVAC lines. We analyze the ion flow field with and without the dedicated metallic return conductor (DMRC). In the absence of wind, the maximum of the electric field is lower than that of the HVAC lines and the current density is very low on the ground. In the presence of wind, the electric field and the current distribution become unsymmetrical on the ground level. The peak current density increases significantly on the downwind side. The HVDC line without DMRC has the electric field peak higher than that of the HVAC lines. With the higher wind shear coefficient, the peak of the electric field and the current density increases on the downwind side. Overall investigations summarize that DMRC can effectively reduce the peak of the electric field to be lower than that of the existing 500 kV HVAC lines under all wind conditions.

It is of interest for companies to keep the annual operating cost of their buildings as low as possible. A substantial share of the annual operating costs are due to the large amount of energy needed for heating of the ventilated air and heating of the rooms inside the buildings. Also much of the electrical energy in the world today is created using fossil fuel or charcoal. This has an environmental aspect and the consumers of energy becomes more and more aware of this. Thus reducing the energy used by a buildings HVAC system can save the users for considerable expenditures and also has an environmental aspect.To find an estimate of the energy consumption a mathematical model representing a building and its HVAC system have been made. This model has been made up of several smaller models representing each component present in the building. These models have then been implemented in verb|Simulink| and the response of the system has been simulated for different scenarios. From these simulations the energy consumption has been extracted and compared to each other. Thus the amount of energy saved for each scenario has been found. The models include two type of controllers to see whether or not the choice of controller design affects the energy efficiency of the system. These two controller designs are the PID controller and the MPC control scheme. Also a discretized and simplified model of the building to be used together with the MPC controller has been found using system identification. In addition to this a Kalman filter that estimates unknown states and filter out disturbances are included in the MPC control scheme.The results from the simulations using a PID controller indicates a possible annual saving of substantial amounts. Thus this report shows that the annual energy consumption in a building can be greatly reduced by introducing simple and relatively cheap modifications to the HVAC system. The results from the simulations using the MPC scheme indicates that even more energy can be saved using this advanced control scheme. However, in order to verify this the MPC controller needs to be fine tuned and several more experiments needs to be reviewed.

With the great possibility of offshore wind power that can be installed in the world seas, offshore wind power is starting to get and important source of energy. The growing sizes of wind turbines and a growing distance to land, makes the choice of transmission alternative to a more important factor. The profitability of the transmission solution is affected by many parameters, like investment cost and power losses, but also by parameters like operation & maintenance and lead time of the system. The study is based on a planned wind farm with a rated power of 1 200 MW and at a distance of 125 km to the connection point. Four models have been made for the transmission network with the technology of HVAC, HVDC and a hybrid of both. The simulation program used is EeFarm II, which has an interface in Matlab and Simulink. The four solutions have been compared technically, with difficulties and advantages pointed out and also economically, with the help of LCOE, NPV and IRR. Costs, power losses and availability of the wind turbines and intra array network are not included in the study. The result of the simulations implies that the HVAC solution is the most profitable with the lowest Levelized Cost of Energy and highest Net Price Value and Internal Rate of Return. The values are 25.11 €/MWh, 387.60 M€ and 15.32 % respectively. A HVDC model with just one offshore converter station, has a LCOE close to the HVAC solution, but with a more noticeable difference in NPV and IRR (25.71 €/MWh, 300.76 M€ and 14.84 % respectively). A sensitivity analysis has been done, where seven different parameters have been changed for analysing their impact on the economic result. The largest impact made was by a change in investment cost and lead times. The results imply that with a structure of the transmission network as for the models, and with similar input data, the break point where a HVDC solution is more profitable than a HVAC solution is not yet passed at a distance of 125 km from the connection point. With an evolving technology in the field of HVDC, a shorter lead time and lower investment cost could mean that a HVDC solution would be more profitable at this distance. Difficulties for a HVAC solution with more cable required, like bigger land usage and cable manufacturing as a bottle neck, could make an important factor tough while making a decision.
Med den stora potentialen hos världens hav, börjar havsbaserad vindkraft bli en betydande energikälla. Den ökande storleken på vindkraftsturbinerna tillsammans med de ökade avstånden mellan vindkraftsparkerna och land, gör att transmissionslösningen blir en mer betydelsefull komponent. Flera olika parametrar kan vara avgörande för transmissionslösningens lönsamhet, som investeringskostnad och effektförluster, men också saker som drift & underhåll och projektets ledtid. Studien är baserad på en planerad vindkraftspark med en märkeffekt på 1 200 MW och på ett avstånd på 125 km till anslutningspunkten. Fyra modeller av transmissionssnätet har gjorts, där tekniken har bestått av HVAC, HVDC samt en blandning av dessa. Simuleringarna har gjort i EeFarm II, ett program baserat på Matlab och Simulink. De fyra modellerna har jämförts tekniskt, med för- och nackdelar poängterade, och även ekonomiskt med hjälp av LCOE, NPV och IRR. Kostnader, effektförluster och tillgängligheten för vindkraftsturbinerna och internnätet i vindkraftsparken är inte inkluderade i studien. Resultaten av simuleringarna visar på att HVAC-lösningen är den mest lönsamma, med lägst Levelized Cost of Energy och högst Net Price Value och Internal Rate of Return. Värdena för dessa är 25,11 €/MWh, 387,60 M€ respektive 15,32 %. En HVDC-lösning med enbart en DC-plattform och likriktarstation för hela märkeffekten, har en LCOE inte långt ifrån HVAC-lösningen, men med en lite större skillnad i NPV och IRR (25,71 €/MWh, 300,76 M€ respektive 14,84 %). För att analysera påverkan av olika parametrar på de ekonomiska mätvärdena, har en osäkerhetsanalys gjort. Den största påverkan på resultatet syntes av förändringar av investeringskostnader och ledtider. Ovanstående resultat tyder på, med transmissionslösningar enligt modellerna i detta arbete, att brytpunkten där en HVDC-lösning är mer lönsam än en HVAC-lösning inte än är passerad vid ett avstånd på 125 km till anslutningspunkten. Med en fortfarande väldigt ung teknik för HVDC, kan den ständigt utvecklande tekniken i framtiden betyda kortare ledtider och en lägre investeringskostnad för en HVDC-lösning och möjligheten att vara en mer lönsam lösning. Komplikationer med en HVAC-lösning pga den extra landkabeln, som större landanvändning och med kabeltillverkningen som en flaskhals, kan ändå göra en HVDC-lösning mer praktisk.

Advances in Information and Communication Technology (ICT) paved the way for decentralized Heating, Ventilation, and Air-Conditioning (HVAC) HVAC operations. It has been envisioned that development of personal thermal comfort profiles leads to accurate predictions of each occupant's thermal comfort state and such information is employed in context-aware HVAC operations for energy efficiency. This dissertation has three key contributions in realizing this envisioned HVAC operation. First, it presents a systematic review of research trends and developments in context-aware HVAC operations. Second, it contributes to expanding the feasibility of the envisioned HVAC operation by introducing novel sensing technologies. Third, it contributes to shedding light on viability and potentials of comfort-aware operations (i.e., integrating personal thermal comfort models into HVAC control logic) through a comprehensive assessment of energy efficiency implications. In the first contribution, by developing a taxonomy, two major modalities – occupancy-driven and comfort-aware operations – in Human-In-The-Loop (HITL) HVAC operations were identified and reviewed quantitatively and qualitatively. The synthesis of previous studies has indicated that field evaluations of occupancy-driven operations showed lower potentials in energy saving, compared to the ones with comfort-aware operations. However, the results in comfort-aware operations could be biased given the small number of explorations. Moreover, required data representation schema have been presented to foster constructive performance assessments across different research efforts. In the end, the current state of research and future directions of HITL HVAC operations were discussed to shed light on future research need. As the second contribution, moving toward expanding the feasibility of comfort-aware operations, novel and smart sensing solutions have been introduced. It has been noted that, in order to have high accuracy in predicting individual's thermal comfort state (≥90%), user physiological response data play a key part. However, the limited number of applicable sensing technologies (e.g., infrared cameras) has impeded the potentials of implementation. After defining required characteristics in physiological sensing solutions in context of comfort-aware operations (applicability, sensitivity, ubiquity, and non-intrusiveness), the potentials of RGB cameras, Doppler radar sensors, and heat flux sensors were evaluated. RGB cameras, available in many smart computing devices, could be a ubiquitous solution in quantifying thermoregulation states. Leveraging the mechanism of skin blood perfusion, two thermoregulation state quantification methods have been developed. Then, applicability and sensitivity were checked with two experimental studies. In the first experimental study aiming to see applicability (distinguishing between 20 and 30C with fully acclimated human bodies), for 16 out of 18 human subjects, an increase in their blood perfusion was observed. In the second experimental study aiming to evaluate sensitivity (distinguishing responses to a continuous variation of air temperature from 20 to 30C), 10 out of 15 subjects showed a positive correlation between blood perfusion and thermal sensations. Also, the superiority of heat flux data, compared to skin temperature data, has been demonstrated in predicting personal thermal comfort states through the developments of machine-learning-based prediction models with feature engineering. Specifically, with random forest classifier, the median value of prediction accuracy was improved by 3.8%. Lastly, Doppler radar sensors were evaluated for their capability of quantifying user thermoregulation states leveraging the periodic movement of the chest/abdomen area induced by respiration. In an experimental study, the results showed that, with sufficient acclimation time, the DRS-based approach could show distinction between respiration states for two distinct air temperatures (20 and 30C). On the other hand, in a transient temperature without acclimation time, it was shown that, some of the human subjects (38.9%) used respiration as an active means of heat exchange for thermoregulation. Lastly, a comprehensive evaluation of comfort-aware operations' performance was carried out with a diverse set of contextual and operational factors. First, a novel comfort-aware operation strategy was introduced to leverage personal sensitivity to thermal comfort (i.e., different responses to temperature changes; e.g., sensitive to being cold) in optimization. By developing an agent-based simulation framework and thorough diverse scenarios with different numbers and combinations of occupants (i.e., human agents in the simulation), it was shown that this approach is superior in generating collectively satisfying environments against other approaches focusing on individual preferred temperatures in selection of optimized setpoints. The energy implications of comfort-aware operations were also evaluated to understand the impact from a wide range of factors (e.g., human and building factors) and their combinatorial effect given the uncertainty of multioccupancy scenarios. The results demonstrated that characteristics of occupants' thermal comfort profiles are dominant in impacting the energy use patterns, followed by the number of occupants, and the operational strategies. In addition, when it comes to energy efficiency, more occupants in a thermal zone/building result in reducing the efficacy of comfort-driven operation (i.e., the integration of personal thermal comfort profiles). Hence, this study provided a better understanding of true viability of comfort-driven HVAC operations and provided the probabilistic bounds of energy saving potentials. These series of studies have been presented as seven journal articles and they are included in this dissertation.
Doctor of Philosophy
With vision of a smart built environment, capable of understanding the contextual dynamics of built environment and adaptively adjusting its operation, this dissertation contributes to context-aware/decentralized HVAC operations. Three key contributions in realization of this goal include: (1) a systematic review of research trends and developments in the last decade, (2) enhancing the feasibility of quantifying personal thermal comfort by presenting novel sensing solutions, and (3) a comprehensive assessment of energy efficiency implications from comfort-aware HVAC operations with the use of personal comfort models. Starting from identifying two major modalities of context-aware HVAC operations, occupancy-driven and comfort-aware, the first part of this dissertation presents a quantitative and qualitative review and synthesis of the developments, trends, and remaining research questions in each modality. Field evaluation studies using occupancy-driven operations have shown median energy savings between 6% and 15% depending on the control approach. On the other hand, the comfort-aware HVAC operations have shown 20% energy savings, which were mainly derived from small-scale test beds in similar climate regions. From a qualitative technology development standpoint, the maturity of occupancy-driven technologies for field deployment could be interpreted to be higher than comfort-aware technologies while the latter has shown higher potentials. Moreover, by learning from the need for comparing different methods of operations, required data schemas have been proposed to foster better benchmarking and effective performance assessment across studies. The second part of this dissertation contributes to the cornerstone of comfort-aware operations by introducing novel physiological sensing solutions. Previous studies demonstrated that, in predicting individual's thermal comfort states, using physiological data in model development plays a key role in increasing accuracy (>90%). However, available sensing technologies in this context have been limited. Hence, after identifying essential characteristics for sensing solutions (applicability, sensitivity, ubiquity, and non-intrusiveness), the potentials of RGB cameras, heat flux sensors, and Doppler radar sensors were evaluated. RGB cameras, available in many smart devices, could be programmed to measure the level of blood flow to skin, regulated by the human thermoregulation mechanism. Accordingly, two thermoregulation states' quantification methods by using RGB video images have been developed and assessed under two experimental studies: (i) capturing subjects' facial videos in two opposite temperatures with sufficient acclimation time (20 and 30C), and (ii) capturing facial videos when subjects changed their thermal sensations in a continuous variation of air temperature from 20 to 30C. Promising results were observed in both situations. The first study had subjects and 16 of them showed an increasing trend in blood flow to skin. In the second study, posing more challenges due to insufficient acclimation time, 10 subjects had a positive correlation between the level of blood flow to skin with thermal sensation. With the assumption that heat flux sensing will be a better reflection of thermoregulation sates, a machine learning framework was developed and tested. The use of heat flux sensing showed an accuracy of 97% with an almost 4% improvement compared to skin temperature. Lastly, Doppler radar sensors were evaluated for their capability of quantifying thermoregulation states by detecting changes in breathing patterns. In an experimental study, the results showed that, with sufficient acclimation time, the DRS-based approach could show distinction between respiration states for two distinct air temperatures (20 and 30C). However, using a transient temperature was proven to be more challenging. It was noted that for some of the human subjects (38.9%), respiration was detected as an active means of heat exchange. It was concluded that specialized artifact removal algorithms might help improve the detection rate. The third component of the dissertation contributed by studying the performance of comfort-driven operations (i.e., using personal comfort preferences for HVAC operations) under a diverse set of contextual and operational factors. Diverse scenarios for interaction between occupants and building systems were evaluated by using different numbers and combinations of occupants, and it was demonstrated that an approach of addressing individual's thermal comfort sensitivity (personal thermal-comfort-related responses to temperature changes) outperforms other approaches solely focusing on individual preferred temperatures. The energy efficiency implications of comfort-driven operations were then evaluated by accounting for the impact of human and building factors (e.g., number of thermal zones) and their combinations. The results showed that characteristics of occupants' thermal comfort profiles are dominant in driving the energy use patterns, followed by the number of occupants, and operational strategies. As one of the main outcomes of this study, the energy saving and efficiency (energy use for comfort improvement) potentials and probabilistic bounds of comfort-driven operations were identified. It was shown that keeping the number of occupants low (under 6) in a thermal zone/building, boosts the energy saving potentials of comfort-driven operations. These series of studies have been presented as seven journal articles, included in this dissertation.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009.
ENGLISH ABSTRACT: Limited research results are available on the aging impacts of surface discharges for High Voltage Direct Current (HVDC) excitation on practical insulators using the Tracking Wheel Tester (TWT) methodology. This thesis gives details of an experimental investigation to compare the aging performance of insulator samples using the TWT for High Voltage Alternating Current (HVAC) and positive and negative polarity HVDC excitation. Two series of tests were performed. The first series of tests evaluated the aging of six insulator rods, namely three Room Temperature Vulcanized Silicone Rubber (RTV SR) coated glass samples and three uncoated glass samples. Three creepage distances were used for each of the test materials, namely 277 mm, 346 mm and 433 mm. The tests were conducted with HVAC excitation and the test methodology described in the IEC 61302 standard. The second series of tests involved the testing of actual insulator samples. The test voltage and conductivity of the salt water solution were also adapted. Six insulators from different manufacturers, representing different materials and specific creepage distances, were tested with HVAC and positive and negative polarity HVDC excitation. The test samples consisted of Ethylene Propylene Diene Monomer (EPDM), High Temperature Vulcanized Silicone Rubber (HTV SR), porcelain and SR coated porcelain insulators. The aging performance of the different test samples for the three types of excitation are compared in terms of peak leakage current, visual observations of surface degradation and hydrophobicity properties. The leakage current data for HVAC excitation shows that the insulators from the different manufacturers perform differently, even for the same type of material. The results also indicate differences in the way the surfaces degrade, as well as the rate of degradation. For insulators representing the same specific creepage distance, but different materials, it has been shown that the material does influence the aging performance. Comparison of the leakage current data for the HTV SR insulators from the same manufacturer, but with different specific creepage distances, shows that the specific creepage distance affect the aging performance significantly. In general, the test insulators showed higher peak leakage currents with HVDC excitation compared to HVAC excitation. The results for positive polarity HVDC excitation show that the dry band arcing, as well as the discharges, has the same form for all six insulators. The colour of the dry band discharges ranged from a blue-ish orange to a dark yellow, depending on the intensity of the leakage currents. The hydrophobic insulators, namely the HTV SR, EPDM and RTV SR coated porcelain insulators, had lost hydrophobicity within the first week of testing. The results for negative polarity HVDC excitation show severe surface degradation compared to the results for HVAC excitation. The effect of positive polarity HVDC excitation, however, seems to be more severe in terms of leakage currents and aging compared to negative polarity HVDC excitation.

This study investigates the power stability problems of HVDC and VSC-HVDC interactions on their hybrid networks with HV AC link, with the intention of bringing out their weaknesses and strengths. The knowledge of this will assist network planners to be informed on ways of improving the efficiency and quality of power systems network. The simulations for this study was done using DIgSILENT Powerfactory software version 14.0.515. This study encapsulates the three major stability problems affecting power systems network, namely, the voltage stability, transient stability and small signal stability. The voltage stability study was conducted using series of load flows at various levels to plot the VP and QV curves, and the results were used to analyze the systems proximity and sensitivity to voltage collapse, as well as the maximum loading point (MPL) of the network. Furthermore, the voltage angle, and terminal voltage responses during a three-phase short circuit disturbance was also used to analyze the voltage stability of the networks. For the transient stability study, several case studies were investigated and their dynamic performances during three-phase short circuit perturbations were analyzed. The small signal stability investigation was done using modal analysis to determine the small signal stability of the three transmission schemes mentioned above. The transient and small signal stability, which are both subsets of rotor angle stability, were further investigated to show the effect of power systems stabilizer (PSS) and automatic voltage regulator (A VR) on rotor angle stability. The results of the analyses show that the HVDC transmission scheme provides the best alternative for bulk power transmission over a long distance. The VSC-HVDC transmission network is suitable for interconnections where the tie with HV AC networks have a low short circuit ratio (SCR). Other conclusions reached with the investigations are explained in chapter ten.

Dynamic simulations have been successfully applied to the modelling of building heating, ventilating and air-conditioning (HVAC) plant operation. These simulations are generally driven using time-series data as input. Whilst time-series simulations are effective, they tend to be expensive in terms of computer execution time. A possible method for reducing simulation time is to develop a probabilistic picture of the model, by characterising the model as being in one of several states. By determining the probability for being in each model state, predictions of long-term values of quantities of interest can then be obtained using ensemble averages. This study aims to investigate the applicability of the Markov modelling method for the above stated purpose in the simulation of HVAC systems. In addition, the questions of the degree of accuracy which can be expected, and the amount of time-savings which are possible are investigated. The investigation has found that the Markov modelling technique can be successfully applied to simulations of HVAC systems, but that assumptions commonly made concerning the independence of driving variables may often not be appropriate. An alternative approach to implementing the Markov method, taking into Z): account dependencies between driving variables is suggested, but requires further development to be fully effective. The accuracy of results has been found to be related to the sizes of the partial derivatives of the calculated quantity with respect to each of the variables on which it depends, the sizes of the variables' ranges, and the number of states assigned to each variable in developing the probabilistic picture of the model's state. A deterministic error bound for results from Markov simulations is also developed, based on these findings.

Currently heating, ventilation and air conditioning (HVAC) systems are difficult and costly to monitor for energy efficiency performance and reliability. As buildings evolve, they will require higher levels of insulation and air tightness which will require ventilation systems to provide the minimum number of air changes and reduced energy usage by recovering heat from the air before it is expelled. This will necessitate the need for monitoring of the operating performance of these systems so that air quality or building energy efficiency is not detrimentally affected. A typical duct airflow monitoring device uses a pressure differential method to determine the airflow rate but they are fragile, expensive and create an additional pressure loss. The monitoring of airflow rates can indicate problems in the design, installation and operation of a HVAC system. One of the possible alternatives to using pressure differential type devices such as Pitot tube/arrays, orifice plates and Venturis is to use an ultrasonic flow rate sensor, but historically their high cost has restricted their use in HVAC systems. This project has looked at improving on existing measuring systems by developing an ultrasonic in-duct flowmeter system to measure the mean airflow, temperature and humidity of a ventilation duct so that a comparative energy level can be accurately deduced. A proof of concept in-duct ultrasonic airflow monitoring device has been developed and has obtained results within ±3.5% RMS of a Venturi airflow measuring device. Matlab code for a Monte Carlo acoustic ray/particle tracing ultrasonic flowmeter simulation has been developed to study the effects of non-ideal installation scenarios. The fully developed centreline computational fluid dynamics (CFD) mean flow velocity to duct total mean flow velocity error can be up to 13%. Analysis of the CFD data for various duct scenarios has shown that this could be reduced to below 5% by using a transducer offset of approximately ±0.25 duct diameters or widths from the centreline at distances as close as one duct hydraulic diameter from an upstream disturbance, such as caused by a bend.

Directional airflow has been utilized to enable targeted air conditioning in cars and airplanes for many years, where the occupants could adjust the direction of flow. In the building sector however, HVAC systems are usually equipped with stationary diffusors that can only supply the air either in the form of diffusion or with fixed direction to the room in which they have been installed. In the present thesis, the possibility of adopting directional airflow in lieu of the conventional uniform diffusors has been investigated. The potential benefits of such a modification in control capabilities of the HVAC system in terms of improvements in the overall occupant thermal comfort and energy consumption of the HVAC system have been investigated via a simulation study and an experimental study. In the simulation study, an average of 59% per cycle reduction was achieved in the energy consumption. The reduction in the required duration of airflow (proportional to energy consumption) in the experimental study was 64% per cycle. The feasibility of autonomous control of the directional airflow, has been studied in a simulation experiment by utilizing the Reinforcement Learning algorithm which is an artificial intelligence approach that facilitates autonomous control in unknown environments. In order to demonstrate the feasibility of enabling the existing HVAC systems to control the direction of airflow, a device (called active diffusor) was designed and prototyped. The active diffusor successfully replaced the existing uniform diffusor and was able to effectively target the occupant positions by accurately directing the airflow jet to the desired positions.
M.S.
The notion of adjustable direction of airflow has been used in the car industry and airplanes for decades, enabling the users to manually adjust the direction of airflow to their satisfaction. However, in the building the introduction of the incoming airflow to the environment of the room is achieved either by non-adjustable uniform diffusors, aiming to condition the air in the environment in a homogeneous manner. In the present thesis, the possibility of adopting directional airflow in place of the conventional uniform diffusors has been investigated. The potential benefits of such a modification in control capabilities of the HVAC system in terms of improvements in the overall occupant thermal comfort and energy consumption of the HVAC system have been investigated via a simulation study and an experimental study. In the simulation study, an average of 59% per cycle reduction was achieved in the energy consumption. The reduction in the required duration of airflow (proportional to energy consumption) in the experimental study was 64% per cycle on average. The feasibility of autonomous control of the directional airflow, has been studied in a simulation experiment by utilizing the Reinforcement Learning algorithm which is an artificial intelligence approach that facilitates autonomous control in unknown environments. In order to demonstrate the feasibility of enabling the existing HVAC systems to control the direction of airflow, a device (called active diffusor) was designed and prototyped. The active diffusor successfully replaced the existing uniform diffusor and was able to effectively target the occupant positions by accurately directing the airflow jet to the desired positions.

Abstract Heating, Ventilation and Air Conditioning (HVAC) control and energy usage management has been identified as a promising way of improving building energy efficiency and thus contributing to solving the energy challenges of the world. However, a critical aspect of reducing HVAC energy usage is fault detection and diagnosis (FDD). Several studies have been conducted on FDD in HVAC systems with less focus on residential HVAC systems. One reason identified for this reduced attention with residential HVAC is that state-of-the-art FDD tools greatly depend on data available through the building automation system (BAS), and this detailed data is not typically available in the residential sector. Meanwhile, using sensors for developing FDD-enabled HVAC systems is not cost-efficient due to the cost associated with required sensors compared to the energy savings realized, thus making residential FDD less attractive. However, studies have shown that faults cause an additional 20.7TWh of energy consumption from residential HVACs across the US, annually. Thus, this paper gives a critical review of various studies that have been done on FDD in residential HVAC systems and proposes a data analytical approach, which if actualized, could reduce the sensor requirements for FDD in residential HVAC, thus addressing the cost barrier.

Abstract As floating wind farms move from pilot projects to commercial-scale installations they will move further offshore and into deeper water. There will be a requirement for offshore substations to deliver the electricity to shore, for which floating support structures will be the preferred solution. This paper describes the challenges and development of solutions for commercial-scale HVAC and HVDC floating offshore substations. Two different floating substation concepts have been developed. Layouts for the electrical and ancillary equipment were initially developed, to enable efficient packaging and structural efficiency for the topsides. By integrating the hull and topsides, the overall mass of the structure is minimised, benefitting stability and reducing hull size. Hydrodynamic analysis of the substructures was performed and structural code checks on the hull and topsides were carried out in Sesam. Mooring designs for each structure for 250m water depth have been developed and analysed in Orcaflex. It is likely that alternating current (HVAC) export to shore will be used for shorter transmission distances and direct current (HVDC) will be used for longer transmission distances. HVDC and HVAC floating substations will have quite different hull forms. The larger topsides footprint and greater mass of the HVDC conversion equipment make a conventional semi-submersible hull form efficient when allied to a stressed-skin topsides structure. The smaller footprint, lighter weight and differing requirements for protection from the elements of the HVAC topsides make this inefficient, so a deep draught semi-submersible with a hybrid topsides is the preferred solution. It is concluded that floating substations suitable for large, commercial-scale wind farms will be the chosen solution for anything other than shallow water or close to shore.

Several potential energy-savings devices are available for through-the-wall HVAC (heating, ventilation, and air-conditioning) systems but independent test data quantifying their savings are sparse. This field evaluation and demonstration study quantifies the potential energy savings and assesses the impacts on occupants and staff of using a supervisory HVAC controller in the lodging industry. The supervisory controller operates according to the occupancy status of each room in a hotel or motel. The controller is a plug-in device installed between a wall outlet and the room HVAC unit. Preliminary tests in laboratory simulated environmental conditions suggest that energy savings ranging from 50% to 80% are possible in unoccupied lodging rooms employing the controller (Fisher, 1999). The field study objective was to quantify and verify these estimates and to identify impacts on occupant comfort. The controller was developed as a tool for reducing energy consumption and lowering operating costs in the lodging industry. This paper describes results from a field evaluation of the performance of a supervisory HVAC controller. During the test period, the uncontrolled rooms HVAC energy consumption averaged 2632.2 kWh monthly while the controlled rooms HVACs consume an averaged 1684.6 kWh, which equates to the uncontrolled rooms HVAC consuming an averaged of 947.6 kWh (56.3%) more energy than the units in the controlled room monthly. Due to intermittent inoperability of the controllers and other energy loads (controllers controlled approximately 50% of the hotel’s conditioned space), comparison analysis of the energy cost over a three-year period for the entire hotel does not reveal any discrete savings during the test period over prior years.