Electromagnetic compatibility of power line communication systems

The power system has been used for communication purposes for many decades, although it was mainly the power utility companies that used low bit rates for control and monitoring purposes. In the last ten years, however, the deregulation of the power and telecommunication markets has spurred the idea of using and commercializing the power networks for a range of new communication applications and services. The idea has been developed and implemented into both, narrowband and broadband systems, which are defined in terms of the operation frequency band. Depending on the frequency band, the systems over powerlines can be: Narrow-band. They use frequencies ranging from 3-148.5 kHz in Europe, with the upper frequency extending up to 500 kHz in the United States and Japan. In Europe, this frequency range is standardized by CENELEC Standard EN 50065. Broadband. The used frequency range is 1-30MHz; 1-15MHz for outdoor systems and 15-30MHz for indoor systems. In this frequency range, the standardization situation is still unclear and there exist no regulations. The developed applications and systems use different parts of the power network: medium voltage (MV) and low voltage (LV) cabling for outdoor applications and building cabling for indoor applications. These cables are designed and optimized for power transmission at frequencies of 50/60Hz and represent a hostile medium for transmissions at higher frequencies. This thesis concentrates on electromagnetic compatibility (EMC) aspects and some optimization issues of the broadband systems, currently known as Powerline Communications (PLC) or Broadband Power Line (BPL). The work presented here was preformed in the framework of the European project OPERA (http://www.ist-opera.org/). A short description of the project is given in Chapter 1. The second chapter presents the basis, introduction, description and state of the art of the topics of interest for this thesis. That chapter is divided into three parts. Each of these parts starts with a short introduction to the topic to be addressed. The introductions are intended for those not familiar with the topic at hand and they can be skipped by those already knowledgeable of it. The first part of Chapter 2 gives an overview and introduction to telecommunication issues relevant to the thesis, as well as the general technical specifications of the OPERA system. The second part deals with the transmission medium which, for the case of PLC, is the power system. The fundamentals and the different components of the PLC system are given there and the state of the art regarding the transmission channel is presented. The third part deals with the EMC and standardization issues related to the technology. The main contributions of the thesis are presented in chapters 3 to 7. The PLC technology distinguishes itself from other technologies in that it uses already existing, ubiquitous wiring, so that no new infrastructure is needed. On the other hand, using a channel designed originally for other purposes means that it is not optimized for the frequencies and applications of interest for broadband transmission. If PLC is to compete with other technologies, these problems have to be well understood and solved, so that the system can be optimized by taking into account the parameters and constrains of the already existing medium. Although the PLC system is being improved continuously, there are still concerns about emissions, immunity and standardization. These issues are important since PLC operates in an environment already populated by other services at the same frequencies, so that fair co-existence is needed. Moreover, the PLC modem has a combined mains and telecom port and, as a consequence, the standards for conducted emissions from those two types of ports are not directly applicable. In addition, the symmetry of the cables used is low and, therefore, emissions are higher than, for example, emissions from twisted pair cables used in xDSL. A good understanding of emissions and immunity in PLC systems is therefore of great importance for the optimization of the system and for EMC standardization to be based on objective technical criteria. Even if the basic phenomena are essentially the same as for any other wire transmission system, the complexity and variability of the topologies of existing structures is so large that simple, straightforward solutions are often not applicable. Emissions from the cabling are primarily due to the common mode signals. Part of the energy in this mode is injected by the imperfectly balanced output stages of the PLC modems themselves. In addition, the common mode appears at punctual imbalanced discontinuities and distributed asymmetry along the PLC signal path in the power cables. Chapter 3 presents the work performed to improve our understanding of the sources of the common-mode current and the parameters that influence its behavior, including related measurements and simulations. For the purpose of this study, a model house was built at the EPFL's test site. Different cablings were used to study the influence of different parameters on the behavior of the common-mode current since it is the main source for both types of emissions, conducted and radiated ones. The influence of different parameters such as the cable terminations, the symmetry of the termination, the height of the conductors above the ground, the presence of power outlets, switches, empty and occupied sockets and the topology, are analyzed. The data are also used to test two methods used to simulate the differential-to-common-mode conversion and the conducted emissions, namely the transmission line model and the full wave approach provided by the Method of Moments through the Numerical Electromagnetic Code (NEC). In Chapter 4, problems related to PLC immunity testing are treated. We show that the conversion of the differential mode to the common mode is coupled with the reverse conversion by reciprocity. Due to the low symmetry of PLC cabling, part of the injected common mode test signal is converted into a differential mode signal that interferes with the wanted signal at the input of the modem being tested. Depending on the actual symmetry of the Coupling-Decoupling Network (CDN), not specified in the standards, the immunity test may yield erroneous results due to the effect of this differential mode component. Working under the assumption that the CDN is built to exhibit a symmetry similar to that of PLC networks as inferred from its longitudinal conversion loss, we estimate the differential mode disturbance level that the modems should withstand from a narrowband interferer. The bit error rate induced by the presence of the disturbing differential mode current from the CDN is also estimated, for a total physical channel transmission rate of 200 Mbps, to be of the order of 1×10-5 to 5×10-5. Since these rates can be handled by error correcting coding and MAC ARQ procedures, it is concluded that the modems are not likely to suffer any severe performance degradation due to immunity testing if the CDN exhibits a symmetry similar to that of PLC networks. Simulating the complete PLC network or any significant part of it using numerical techniques such as the method of moments proves to be of limited practical use due to the fact that PLC networks extend over many wavelengths. The transmission line approximation, on the other hand, although more efficient and sufficiently accurate for differential mode calculations, is not directly applicable to simulate the EMC behavior since it neglects the antenna-mode currents that are significant contributors to the radiated emissions. Chapter 5 presents a novel approach to evaluate the antenna-mode currents using a modified transmission line theory, thus making this numerically efficient technique applicable to the estimation of emissions in PLC. An integral equation describing the antenna-mode currents along a two-wire transmission line is derived. It is further shown that, when the line cross-sectional dimensions are electrically small, the integral equation reduces to a pair of transmission line-like equations with equivalent line parameters (per-unit-length inductance and capacitance). The derived equations make it possible to compute the antenna mode currents using a traditional transmission line code with appropriate parameters. The derived equations are tested versus numerical results obtained using NEC and reasonably good agreement is found. Another important EMC issue related to PLC is the mitigation of emissions. Chapter 6 describes a technique that has been proposed to achieve a reduction of emissions associated with indoor PLC networks through the introduction of a 180° out-of-phase replica of the PLC signal into the unused neutral-ground circuit. A modification to this technique is proposed based on the selection of the appropriate amplitude and phase of the auxiliary signal, allowing a higher degree of field attenuation. A way of implementing this technique is proposed and studied, namely the integration of a required antenna into the PLC modems themselves. The measured fields very close to the modem allow the determination of the magnitude and phase of the compensation voltage. The proposed implementation should be used only to handle customer complaints, when emissions should be lowered at locations where PLC signals might cause unwanted interference or when additional capacity is required and it can be obtained through the gained signal to noise margin. Although, in principle, due to nonalignment of the wanted and the compensation field directions, minimizing one component of the field may result in an increase of the other components, we show that the application of the technique results in an overall average reduction of 10-20 dB of all the field components in the region of interest. In the same Chapter 6, we address the more general issue of the application of mitigation techniques' gained emissions margin to increase the overall throughput of PLC systems. We show that an increase in the signal power (made possible by the inclusion of mitigation techniques) leads to a considerable increase in the PLC channel capacity. Using a number of simplifications, we show that the capacity of the channel can indeed be increased by up to 66 Mbps for mitigation efficiencies of only 10 dB. We also present the results of laboratory measurements aimed at studying, under controlled conditions, different characteristics of notching in OPERA PLC modems, such as total and effective notch width, notch depth, maximum notch depth, etc. These measurements show that it is possible to obtain attenuations of up to about 45 dB for notches in all frequency bands, 10MHz, 20MHz and 30MHz. What differs for these three bands is the minimum number of carriers that need to be notched to obtain that maximum attenuation. This is an important point, since, to implement notches that have the required depth and width, one must know how many subcarriers to suppress and how deep these need to be reduced. High density PLC deployment requires the increase of overall system data rate. To achieve the higher data rates, frequency reuse in these systems is needed. In Chapter 7, we present the idea for using so-called blocking filters as a possible solution for a frequency reuse. Experimental data obtained on a real distribution network show that the use of blocking filters can, in certain cases, ensure high enough RF separation of the LV feeders belonging to the same substation. In some cases, even with the possibility to design and integrate effective blocking filters, the system needs to provide additional synchronization mechanisms for frequency reuse

Intelligent distribution network design

Distribution networks (medium voltage and low voltage) are subject to changes caused by re-regulation of the energy supply, economical and environmental constraints more sensitive equipment, power quality requirements and the increasing penetration of distributed generation. The latter is seen as one of the main challenges for today’s and future network operation and design. In this thesis it is investigated in what way these developments enforce intelligent distribution network design and new engineering tools. Furthermore it should be investigated how a new design and control strategy can contribute to meet the power quality and performance requirements in distribution networks in future. This thesis focuses on network structures that, typical for the Netherlands, are based on relatively short underground cables.Managing current and voltage in such networks both during normal and disturbed operation, requires a good network design and an adequate earthing concept. The limited size of Dutch distribution networks has a positive effect on power quality aspects and reliability. The use of impedance earthing for medium voltage (MV) cable networks reduces the risk of multi-phase faults that cause large fault currents and deep dips. It also reduces the risk on transient overvoltages due to re-striking of cable faults. A TN earthing system for the low voltage (LV) network reduces the risk of damaged apparatus and it maintains safety for people. However, care must be taken for the earthing of devices of other service providers, which requires a co-operative solution. The fast developments of computation techniques and IT equipment in the network opened the possibility to perform many calculations in short time based on both actual and historical data. Examples are the on-line distribution loadflow and the short-circuit calculation for protection coordination and intelligent fault location. In LV and MV network calculations the accuracy of the models and the availability of data are the main obstacles. Because of the unsymmetrical nature of load and generation in LV networks a multiple conductor model is needed. For safety calculations also the earth impedances have to be modelled as well as the neutral and protective earth impedances and their mutual interactions. The protection philosophy in MV networks must take into account the changing requirements regarding safety and power quality. An overall philosophy concerning both network and generator protection is necessary. New developments in substation automation benefit future upgrade and refurbishment of substation control and protection. As a result, also cheap,accurate and fast fault location becomes feasible, reducing the outage time of the customers. Next the influence of distributed generation on the above subjects is investigated. The increasing magnitude of short-circuit currents and the increasing voltage variations in the network are seen as a major challenge for the network planners. Conventional measures for reducing voltage problems may introduce problems with the short-circuit current level and vice versa. In networks which contain a large amount of both load and distributed generation, adverse voltage problems may occur, especially when the generation is located in the LV network. In order to reduce this, specific control strategies need to be developed. The last part of the thesis is related to these control strategies as a solution for operating future distribution networks. By introducing storage and power electronics, networks can be transformed into autonomously controlled networks. These networks remain an inseparable part of the electricity network but may behave in a fairly autonomous manner, both internally and externally, with respect to the rest of the network. The focus in this thesis is on maintaining an optimal voltage for all customers during all combinations of load and generation. Because of the autonomous behaviour of the control systems, their operation must be based on local measurements. A suggested approach is to replace the normal open point between MV feeders by a so called "intelligent node". This node is able to control the power flow in several feeders by means of power electronics and, if provided, by electricity storage. The voltage profile can be improved further, by introducing an intelligent voltage control on the HV/MV transformer feeding the distribution network. The simulation studies in this research have been performed on a realistic model of a typical Dutch MV/LV distribution system. Based on the results the following conclusions are drawn: • The HV/MV transformer control must be based on line drop compensation. This compensation must use the load situation instead of the measured exchange signal. The compensation factor must differ between cases of high load and of high generation. • The optimal control of the intelligent node is a voltage control, based on a linear dependence of the voltage at the node and the power flow towards that node. This method can be improved when the voltage of the MV bus bar in the substation is taken into account. • Methods to obtain a perfect voltage profile will lead to a storage device that is not available for this voltage level yet. • A voltage control based on a fixed value at both terminals of the intelligent node and at the MV bus bar of the HV/MV substation does not result in the optimal voltage profile, although guarantee a good voltage quality and might therefore be a good alternativ

Intelligent voltage dip mitigation in power networks with distributed generation

Includes bibliographical references.The need for ensuring good power quality (PQ) cannot be over-emphasized in electrical power system operation and management. PQ problem is associated with any electrical distribution and utilization system that experiences any voltage, current or frequency deviation from normal operation. In the current power and energy scenario, voltage-related PQ disturbances like voltage dips are a fact which cannot be eliminated from electrical power systems since electrical faults, and disturbances are stochastic in nature. Voltage dip tends to lead to malfunction or shut down of costly and mandatory equipment and appliances in consumers’ systems causing significant financial losses for domestic, commercial and industrial consumers. It accounts for the disruption of both the performance and operation of sensitive electrical and electronic equipment, which reduces the efficiency and the productivity of power utilities and consumers across the globe. Voltage dips are usually experienced as a result of short duration reduction in the r.m.s. (r.m.s.- root mean square) value of the declared or nominal voltage at the power frequency and is usually followed by recovery of the voltage dip after few seconds. The IEEE recommended practice for monitoring electric power quality (IEEE Std. 1159-2009, revised version of June 2009), provides definitions to label an r.m.s. voltage disturbance based upon its duration and voltage magnitude. These disturbances can be classified into transient events such as voltage dips, swells and spikes. Other long duration r.m.s. voltage variations are mains failures, interruption, harmonic voltage distortion and steady-state overvoltages and undervoltages. This PhD research work deals with voltage dip phenomena only. Initially, the present power network was not designed to accommodate renewable distributed generation (RDG) units. The advent and deployment of RDG over recent years and high penetration of RDG has made the power network more complex and vulnerable to PQ disturbances. It is a well-known fact that the degree of newly introduced RDG has increased rapidly and growing further because of several reasons, which include the need to reduce environmental pollution and global warming caused by emission of carbon particles and greenhouse gases, alleviating transmission congestion and loss reduction. RDG ancillary services support especially voltage and reactive power support in electricity networks are currently being recognized, researched and found to be quite useful in voltage dip mitigation

Telecommunications networks for remote electricity supply metering and load control

The aims and objectives of this thesis are to investigate remote electricity supply metering and load control in terms of the now availble UK Electricity Supply Industry (ESI) private and national telecommunications networks, the intelligent building, the home computer and domestic energy management concepts. This work commences with an overview of private telecommunications systems utilised within the U.K. electricity supply industry together with those network services provided by Public Telecommunications Service Operators (PTO's) for customer access (Chapters 1 and 2). The thesis continues by describing the meter reading and billing processes (Chapter 3) and introduces the concepts of remote metering, the consumer billing interface (Chapters 4 and 5), load control and spot pricing theory (Chapter 6). A review of recent load control and remote metering field trials, conducted in the UK, including feasibility studies are then detailed (Chapter 7). A mathematical analysis of two basic approaches to the principle of 'idle-line' working is also considered (Chapter 7). The 'intelligent home' concept and the customer billing interface are then considered in conjunction with the development of a 'home computer' applications strategy (Chapter 8). The development of text, communications and control simulation on the BBC microcomputer, are then detailed by reference to the 'Adaptive Microprocessor based System for Experimentation in the Transmission of Text' (AMTEXT) developed to test the feasibility of the home computer applications strategy developed in Chapter 8 (Chapter 9). The concept of 'idle-time working is then introduced coupled with the concept of 'integration' by way of the national telecommunications network services. Proposals for a Modular Integrated Data Aquisition System (MIDAS) are then considered as a means of illustrating a practical application of both integration and idle-time working (Chapter 10). The thesis continues by considering network integrity, security and reliability in terms of network architecture and the development of a strategy for quantifying network resilience as a design parameter (Chapter 11). Finally, the thesis concludes by summarisirig the work undertaken and the results obtained with respect to the initial objectives, and details potential areas for further research

Inductive interconnecting solutions for airworthiness standards and power-quality requirements compliance for more-electric aircraft/engine power networks

Driven by efficiency benefits, performance optimization and reduced fuel-burn, the aviation industry has witnessed a technological shift towards the broader electrification of on-board systems, known as the More-Electric Aircraft (MEA) concept. Electrical systems are now responsible for functions that previously required mechanical, hydraulic or pneumatic power sources, with a subset of these functions being critical or essential to the continuity and safety of the flight.;This trend of incremental electrification has brought along benefits such as reductions in weight and volume, performance optimization and reduced life-cycle costs for the aircraft operator. It has however also increased the necessary engine power offtake and has made the electrical networks of modern MEA larger and more complex. In pursuit of new, more efficient electrical architectures, paralleled or interconnected generation is thought to be one platform towards improved performance and fuel savings.;However, the paralleling of multiple generation sources across the aircraft can breach current design and certification rules under fault conditions. This thesis proposes and evaluates candidate interconnecting solutions to minimize the propagation of transients across the interconnected network and demonstrates their effectiveness with reference to current airworthiness standards and MIL-STD-704F power quality requirements.;It demonstrates that inductive interconnections may achieve compliance with these requirements and quantifies the estimated mass penalty incurred on the electrical architecture, highlighting how architectural and operating strategies can influence design options at a systems level. By examining the impact of protection operation speed on the electrical network, it determines that fast fault protection is a key enabling technology towards implementing lightweight and compliant interconnected architectures.;Lastly, this thesis addresses potential implications arising from alternate standards interpretations within the framework of interconnected networks and demonstrates the impact of regulatory changes on the electrical architecture and interconnecting solutions.Driven by efficiency benefits, performance optimization and reduced fuel-burn, the aviation industry has witnessed a technological shift towards the broader electrification of on-board systems, known as the More-Electric Aircraft (MEA) concept. Electrical systems are now responsible for functions that previously required mechanical, hydraulic or pneumatic power sources, with a subset of these functions being critical or essential to the continuity and safety of the flight.;This trend of incremental electrification has brought along benefits such as reductions in weight and volume, performance optimization and reduced life-cycle costs for the aircraft operator. It has however also increased the necessary engine power offtake and has made the electrical networks of modern MEA larger and more complex. In pursuit of new, more efficient electrical architectures, paralleled or interconnected generation is thought to be one platform towards improved performance and fuel savings.;However, the paralleling of multiple generation sources across the aircraft can breach current design and certification rules under fault conditions. This thesis proposes and evaluates candidate interconnecting solutions to minimize the propagation of transients across the interconnected network and demonstrates their effectiveness with reference to current airworthiness standards and MIL-STD-704F power quality requirements.;It demonstrates that inductive interconnections may achieve compliance with these requirements and quantifies the estimated mass penalty incurred on the electrical architecture, highlighting how architectural and operating strategies can influence design options at a systems level. By examining the impact of protection operation speed on the electrical network, it determines that fast fault protection is a key enabling technology towards implementing lightweight and compliant interconnected architectures.;Lastly, this thesis addresses potential implications arising from alternate standards interpretations within the framework of interconnected networks and demonstrates the impact of regulatory changes on the electrical architecture and interconnecting solutions

Aspects and directions of internal arc protectio

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