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EMTDC is a nonreal time transient simulator designed by the Manitoba HVDC Research Centre. It incorporates a user-friendly, CAD style interface called PSCAD that allows the user to model a network by interconnecting pre-defined graphical icons that represent the components of the power network. This approach ensures that it is unnecessary to write any direct code, provided the existing models within the EMTDC libraries are acceptable. However, this does not preclude the user from defining their own custom models as graphical icons in an EMTDC component library. This paper indicates how the relative flexibility and userfriendliness of EMTDC, has allowed a new protection for wind farms to be modelled and tested within a simulator.

A variety of FACTS devices are now being proposed as a means of increasing the capability and flexibility of power systems. The planning and design stage of the implementation of this new technology will require reliable and easy-to-use models to represent the FACTS devices and associated networks. The type of modelling needed will depend on the studies being undertaken but will include detailed electromagnetic transient simulations as well as simpler representations. In this paper, digital and physical models for the advanced static VAr compensator (ASVC) and a digital model for the thyristor controlled series capacitor (TCSC) are presented. The detailed models included representations of the power electronic switching circuits, the control systems and the power system network.

Traditionally, security constrained economic dispatch is implemented by adding constraints to the plain economic dispatch-typically limits on line flows and nodal voltages for all credible post-disturbance configurations. However, advances in power electronics, and in particular the concept of FACTS are now challenging conventional approaches to system security operation, planning and overall economy of power systems. Based on FACTS devices, some part: of the security of the system can be achieved in post contingency mode, after an outage occurs. Such an economic dispatch, that incorporates the abilities of FACTS devices to assist in post-fault rearranging of transmission system power flows, allows the system to be operated with relaxed security most of the time. This results in reduction of the system operating costs without actually jeopardising the level of system security. In the context of the England and Wales electricity market, this paper demonstrates how FACTS devices can be used in uplift control. The paper proposes a method to the solution of the economic dispatch problem with an optimal balance between the level of security achieved in preventive and in corrective mode. The method is based on mathematical programming decomposition techniques that allow the iterative solution of a base-case economic dispatch for the intact system and separate contingency analysis with post-contingency corrective control by FACTS devices used to eliminate line/transformer overloads. A case study on a test system is presented and discussed.

This paper considers the impact of embedded generators on the power quality of distribution networks. The various types of embedded generation are discussed and their impact on the network reviewed. The potential for improvement of network power quality using embedded generators is addressed and the barriers to implementation briefly considered.

Battery charging of electric vehicles (EVs) will increase the power demand in distribution networks. It is anticipated that this will cause voltage drops, thermal overloads and an increase in losses. The severity of the impact will depend on the EV owners' behaviour. A generic three-phase low voltage (LV) residential distribution network model was used to evaluate the effects of EV battery charging on distribution networks, the effect of a smart charging regime for EVs as well as the contribution of dispersed micro-generation (mGen) sources. The uncertainties associated with the residential loads, m-Gen and EV owners' behaviour, were addressed with a probabilistic approach. A case study was performed for the year 2030 using both deterministic and probabilistic approaches. The probabilistic assessment showed the importance of the above uncertainties.

Fault ride through of fully rated converter wind turbines in an offshore wind farm connected to onshore network via either high voltage AC (HVAC) or high voltage DC (HVDC) transmission is described. Control of the generators and the grid side converters is shown using vector control techniques. A de-loading scheme was used to protect the wind turbine DC link capacitors from over voltage. How de-loading of each generator aids the fault ride through of the wind farm connected through HVAC transmission is demonstrated. The voltage recovery of the AC network during the fault was enhanced by increasing the reactive power current of the wind turbine grid side converter. A practical fault ride through protection scheme for a wind farm connected through an HVDC link is to employ a chopper circuit on the HVDC link. Two alternatives to this approach are also discussed. The first involves de-loading the wind farm on detection of the fault, which requires communication of the fault condition to each wind turbine of the wind farm. The second scheme avoids this complex communication requirement by transferring the fault condition via control of the HVDC link to the offshore converter. The fault performances of the three schemes are simulated and the results were used to assess their respective capabilities.

An assessment on the capability of a doubly fed induction generator (DFIG) wind turbine for frequency regulation is presented. Detailed aerodynamic, structural and electrical dynamic models were used in this study. A control loop acting on the frequency deviation was added to the inertia contributing loop in order to enhance the inertia support from the DFIG wind turbine. The possibility of de-loading a wind turbine to provide primary and secondary frequency response was discussed. A frequency droop controller was examined where the droop is operating on the electronic torque set point below its maximum speed and is operating on the pitch demand at maximum speed. It is also shown that by reducing the generator torque set point the DFIG wind turbine can provide high frequency response.

A large penetration of wind generation info the power system will mean that poor power quality and poor stability margins cannot be tolerated from wind farms. This requires that methods to improve power qualify and stability for such systems be found. The static compensator (STATCOM) with hybrid battery energy storage (BES) has great potential to fulfil this role, though considerable advances in the control of this system are still to be made. From an economic point of view, rating the STATCOM for steady-state power-quality improvement duty is appropriate. Rating the STATCOM to absorb large amounts of additional power in excess of its transient overload capability during network faults is inappropriate. A hybrid of BES and braking resistor is therefore proposed. A new hybrid STATCOM–BES control technique is developed and discussed in the context of improving the stability and power quality to fixed speed, induction generator, wind turbines. The variation of the network voltage, active and reactive power with the fluctuation of the wind generation is studied. A wind generation system with a STATCOM battery energy storage unit and the new control was simulated and the results demonstrate that both power quality and the stability margin can be improved significantly for wind farms.

A control strategy that provides a doubly-fed induction generator (DFIG)-based wind farm with the capability to provide short-term frequency regulation is investigated. The controller manipulates dynamically the position of the DFIG rotor flux vector to slow down the generator allowing for a temporary surge in the power output, which helps to reduce the frequency drop following the transient period after the loss of network generation. A generic network that combines synchronous and wind-farm generation has been modelled and used for control system design and dynamic assessment. Studies are conducted for the DFIG operating at different speed and power output conditions. Simulation results are described which illustrate the contributions to frequency support of DFIG-based wind farms controlled with the proposed strategy. Machine data and control parameters are provided.

State estimation is important for the automatic management and control of complex distribution networks with significant distributed generation. State estimation has been used extensively on transmission systems where, generally, measurements of busbar voltages and line power flows exist. However, distribution systems normally have only a limited number of measurements. In such systems additional measurements are expensive and careful selection of location becomes important. The paper presents a heuristic approach to identify potential points for location of voltage measurements for state estimation as part of a proposed distribution management system controller. The developed technique identifies measurement locations to reduce the voltage standard deviation of the busbars which do not have a measurement. It addresses the problems of classical transmission meter placement methods, which are not directly applicable to distribution systems due to limited measurements, and unobservability of the network.

The connection of generation to distribution networks may cause problems with existing protection schemes, while development of new protection schemes can facilitate an increase in generation capacity. The design and evaluation of a distribution network protection scheme based on distance relays is described. The scheme provides primary and back-up protection of 11 kV circuits. In the UK, voltage transformers and communication schemes are seldom installed on 11 kV circuits. Hence the distance relay restraint signal (voltage) is measured at the secondary winding of a distribution transformer (11/0.433 kV) to reduce costs. The development of the distance relay algorithm is discussed and a selection of simulation test results provided to illustrate the relay performance under fault conditions.

The impact of system issues on the performance and power rating of dynamics voltage restorers is examined. Algorithms to restore a network voltage sag or swell to given set points have been the subject of significant research in the past. The supervisory control scheme is examined to determine these set points. The control scheme's effect on device power rating (cost) and its interaction with active loads are investigated using analytical methods, simulation and experimental tests. The influence of the choice of the supervisory control algorithm on the selection of protection scheme and the interaction between the protection scheme and the device power rating are also examined.

Present distribution network voltage design practice limits the distributed generation capacity that can be connected to 11 kV networks. It has been shown previously that adoption of a more active approach to network voltage control can significantly increase distributed generation capacity. One way to do this is to control the target voltage of automatic voltage control relays at primary substations. A basic design for a controller to do this has been created, comprising three algorithms. A statistical state estimation algorithm estimates the voltage magnitude at each network node supplied by the primary substation, using real-time measurements, network data and load data. The estimate accuracy depends on the number and placement of real-time measurements. Studies using an 11 kV feeder model showed that an acceptable accuracy could be obtained with one or two measurements. The state estimator uses pseudo measurements for unmeasured loads. A load model was constructed using load profiles to calculate the pseudo measurements. The calculated pseudo measurements were inaccurate, but it was found that acceptable state estimate accuracy could be obtained with inaccurate pseudo measurements. A control algorithm alters the AVC relay target voltage, based on the maximum and minimum node voltage magnitude estimates. A simulation on a four-feeder network showed that the algorithm enabled the generator power export to be more than doubled.

The modelling of a doubly-fed induction generator (DFIG) wind turbine in large power system dynamic simulation software is discussed, and the steady state equivalent circuit and reduced order dynamic DFIG models are described. A direct solution method for obtaining injected rotor voltages, which satisfy the dynamic model control objectives, is derived from the approximate steady state model. An initialisation procedure is given for the reduced order dynamic DFIG machine model. Simulation results are presented from initialised dynamic DFIG wind turbine models. PQ and PV bus representations of the DFIG for load flow studies are compared and simulated with typical turbine and network data for wind farm installations.

With advances in energy storage technology the application of STATCOMs with energy storage for utility applications, such as active- and reactive-power compensation of loads, network-voltage control and mitigation of power system disturbances, is increasingly feasible. As it is more expensive to produce active power than reactive power, it is important to consider methods which can be adopted to minimise the use of the energy store. An application study of a STATCOM with energy storage giving special emphasis to control strategies which minimise the use of the stored energy is reported. Calculation techniques to determine the current rating of the IGBTs, diodes and connecting transformer as well as the losses associated with the switches when the compensator is operated under space-vector modulation are demonstrated. Application studies of the compensator with energy storage for load compensation, steady-state voltage control, mitigation of voltage sags and elimination of power oscillations are described. The analytical studies of each of these applications are supplemented by simulation results carried out in PSCAD/EMTDC and by experimental results obtained from a laboratory prototype.

The dynamic modelling of large (MW) capacity fixed and variable speed induction generator wind turbines is discussed. A reduced order dynamic machine model is derived suitable for modelling both fixed speed and doubly-fed asynchronous generator wind turbines. Control schemes for variable speed turbines, using doubly-fed induction generators (DFIG), are described and simulated. Speed control characteristics and converter protection of the DFIG are implemented in the model. The operation of the models during power system disturbances such as network voltage sags and three-phase faults, as well as the possibility of network voltage instability, are investigated. Simulation results are presented using typical turbine and network data for wind farm installations.

The generation of electrical power using sustainable sources of energy is developing rapidly. The authors describe how the security and quality of electricity supply might be maintained. They discuss the control of variable speed using doubly fed induction generators (DFIG) in wind turbines. The DFIG is constructed from a wound rotor asynchronous machine. Variable speed operation is obtained by injecting a variable voltage into the rotor at slip frequency The injected rotor voltage is obtained using two AC/DC insulated gate bipolar transistor based voltage source converters (VSC), linked by a DC bus. The converter ratings determine the variable speed range. Speed control for optimum power, system frequency control, and machine inertia are also discussed.

Two electrical models of a lead-acid battery, a short-term discharge model and a long-term integrated model, were used to investigate the system performance of a battery-supported dynamic voltage restorer (DVR). The short-term model provides a simple but effective description when the DVR compensates voltage sags over a short period. The integrated model can predict accurately the terminal voltage, state of charge, battery capacity and gassing current. It gives a good description of the battery response during both discharge and charge. Parameters of both models can be determined easily from measured battery output voltages obtained from load-step tests. Both models were successfully implemented in EMTDC/PSCAD and interfaced with the digital model of a 10 kVA DVR physical prototype. They gave a very close agreement between extensive experimental data and simulation results. Application issues such as current harmonics and microcycles during charge/discharge are discussed with respect to their impact on loss of capacity and reduced lifetime of the lead-acid battery.

Control of a dynamic voltage restorer (DVR) based on Space Vector PWM is described. The control algorithm is able to compensate for any type of voltage sag and uses a software phase-locked loop to track phase jumps during a fault. The control algorithm restores the depressed voltages to the same phase and magnitude as the nominal pre-sag voltages and then gradually tracks to the phase of the depressed voltages. Experimental results are shown to validate the control algorithm using a three-phase prototype with a power rating of 10 kVA.

Details of a converter and control system used in a photovoltaic AC module is presented. The sizing of the DC link capacitor, the DC:DC converter design and the maximum power point algorithm are discussed. An overview of system integration including the converter and its control is given. Throughout, the choices of system components are supported with analysis and simulation, as well as experimental tests using a solar emulator test system.

Textbooks get you honours. I am convinced that the vast majority of students can pass even final medical exams without any recourse to books. They certainly don't need reference to those weighty tomes that seem only ever to be found on dusty shelves in medical school libraries. Perhaps the odd vade mecum such as the Oxford Handbook of Medicine and the British National Formulary are almost essential to a good student, but people pass exams without them. The …

At a time when the media are all talking about the flood of incompetent doctors, the stitch-up of the NHS by consultants, and medics as butchers and murderers, what school leaver in their right mind should want to join such a charlatan profession? On the other hand, there is the lifesaving ER image, with gorgeous doctors rushing to the aid of distressed damsels. Fast cars, boundless wealth, and the status of a flourishing Harley Street practice may be attracting them in droves. Reading Learning Medicine will not make readers' decisions for them, but it will certainly leave them with a much clearer idea about what the training and …

Undergraduates in obstetrics and gynaecology have been crying out for a new textbook for years. This was one of the subjects where there was nothing that was quite suited to the needs of medical students. Books for the Diploma Royal College of Obstetrics and Gynaecology were all words, no pictures, and a bit more than we needed to know. Others, such as Ten Teachers, did not always contain enough detail. Then came Impey, and, in the manner of the London bus, James et al arrived at the same time. Suddenly there are two good texts for undergraduates. Impey's book is my favourite - 100 pages of gynaecology followed by 120 pages of obstetrics. All the big topics are covered systematically, and plenty of boxes provide the answer to the “five causes of vaginal …

The commercial viability of wind-driven embedded renewable generation (ERG) projects is sensitive to connection costs. These costs vary with the voltage level at which the ERG is connected; generally the higher the voltage, the higher the cost. Windfarm developers of ERG therefore prefer to connect at lower voltages. However, because distribution networks are normally limited by permissible voltage fluctuations rather than by thermal considerations, network operators prefer to connect ERG at higher voltages where their overall impact on voltage profiles is minimal. This conflict of objectives between ERG developers and network operators is usually settled through simple deterministic load flow studies, usually based on one critical case representing conditions of minimum load and maximum ERG output. Such studies only consider extreme events and do not take into account the likelihood of these events occurring. However, new European standards describe voltage characteristics in statistical terms. Although these standards are not intended for use in assessing the level of penetration of ERG, they point to statistical and probabilistic approaches as the way forward in such assessments. This is particularly important for wind generation because of its stochastic output. Two simple wind speed models based on the Markov modelling technique are presented, which may be used to evaluate the impact of windfarms. The models give satisfactory results when applied to the evaluation of voltage profiles in distribution networks. The probability density function of voltage can be obtained and used in assessing the level of ERG that might be accepted on an existing network. A 281-node generic distribution system model based on a real network is used to illustrate the application of the proposed models.

Deficiencies in present-day loss allocation practices are demonstrated using as an example the substitution method applied in England and Wales to evaluate the impact of embedded generation (EG) on losses. Two new loss allocation schemes are proposed; one based on the allocation of marginal losses and the other on the allocation of total losses. Loss allocation factors computed from the proposed schemes are specific to location and time of use. This is especially important for EG whose impact on losses varies in time and space. A notable feature of the proposed loss allocation coefficients is that they can be positive or negative and therefore can recognise the presence of counter-flows such as those due to the presence of EG. Application of the proposed loss allocation schemes is illustrated on a real network based 265-node generic distribution system model incorporating EG.

An advanced static VAr compensator (ASVC) employing a three-level inverter is presented. The paper describes the operating principles and construction of a hardware model of this ASVC scheme. The performance of the ASVC is obtained from an experimental study carried out on the laboratory model. The use of the selective harmonic elimination modulation technique (SHEM) to minimise harmonics is explored. Experimental studies have been carried out to determine the effect of the DC-side capacitor on the harmonic performance of the scheme and the open-loop response speed is also evaluated. Finally, the economic feasibility of this scheme is briefly assessed by comparing the ASVC with a conventional thyristor-controlled reactor/fixed-capacitor scheme.

In obstructing carcinomas of the left colon, primary resection with immediate colocolonic anastomosis is associated with a high risk of anastomotic dehiscence, and therefore, it has become traditional to manage such patients with staged resection. In obstructing carcinomas of the right colon, immediate resection of the lesion with ileocolonic anastomosis is acceptable. The scope of right hemicolectomy can be extended to encompass obstructing carcinomas of the left colon by performing subtotal or total colectomy with ileosigmoid or ileorectal anastomosis. Herein, we have reported the outcome in 16 patients treated in this manner. The operative mortality was 12.5 percent, which compared favorably the mortality of other methods of surgical management. Extended right hemicolectomy achieves in one operation relief of intestinal obstruction, tumor resection, restoration of gut continuity, elimination of the risks of synchronous or metachronous colonic tumors, and a possible increase in the 5 year survival rate. In our experience, the frequent passage of liquid stools is not a problem. This approach justifies further study.

Energy, exergy and exergoeconomic analysis are often used for assessing large energy conversion systems. However exergy and exergoeconomic analysis are rarely used when small or medium scale energy generation systems, such as community CHP/CCHP plants or microcogeneration systems are evaluated. In this study energy, exergy and exergoeconomic analysis of four Community Energy Supply (CES) systems has been carried out. Biomass Steam Turbine CHP (BST), Gas Turbine CHP (GT), Biomass Integrated Gasification Gas Turbine CHP (BIGGT) and Biomass Integrated Gasification Combined Cycle CHP (BIGCC) systems have been modelled. Modelling and energy/exergy analysis have been conducted using the computer programme Cycle-Tempo. Exergoeconomic evaluation of CESS has been performed using the Specific Exergy Costing (SPECO) approach. Exergy costs of the main products: heat and electricity, have been calculated. The analysis shows that gasification of biomass reduces overall system efficiency due to the exergy destruction in the thermo-chemical conversion process when air is used as an oxidizer. A GT using natural gas as a fuel and BIGCC are the most exergy efficient systems in this study with the lowest exergy cost of electricity and heat produced. The exergy cost of electricity generated in BST is the highest.

Offshore wind power generation is expected to increase in the following years, but there are still some economic and technical challenges to overcome. Because of the difficult access to the offshore facilities, the reduction of maintenance is an essential point. The use of Permanent Magnet Synchronous Generators (PMSG) is considered a suitable option in this wind farm topology to satisfy this purpose. On the other hand, these generators along with full-rated Voltage Source Converters (VSC) are expected to provide ancillary services for the onshore AC grid. Particularly, Inertial Frequency Response is an interesting option considering the stored kinetic energy in the rotor of the wind turbines. Therefore, in this paper a description of the model and control system of a PMSG using full-rated VSC are reviewed and their Inertial Frequency Response capability is presented through two methodologies. At the end, simulation and experimental tests results are shown in order to compare these two different options and analyse their viability.

The function of distribution systems is now changing considerably with the penetration of embedded generation (EG). The main requirement is now not only the distribution of electricity imported from the transmission system and delivered to the end customers, but also in transportation of electricity from EG to the local customers or even exporting it to the transmission system. This development, together with the decoupling of the distribution and supply functions of the business, is opening conceptual and methodological questions related to distribution system operation, planning and pricing. This paper discusses general economic and technical aspects of the interaction between embedded generation and distribution networks. It emphasises both the need for a revision of the ideas behind distribution system operation and reinforcement, and the requirement for the development of new methods and techniques to support emerging technical and administrative changes. (5 pages)

A modelling framework was developed for characterising the impacts of variability and uncertainty of renewable generation and load on a multi-vector and multi-scale energy system. A time series synthesis algorithm was proposed to produce a large number of daily profiles for wind, PV and load, representing possible variation features. Based on the synthetic time series, four models with different uncertainty characteristics were built and applied to generate forecast scenarios for wind and PV. Using the generated scenarios, the operation of a combined gas and electricity system was formulated as a two-stage stochastic mixed-integer linear optimisation problem. A simplified Great Britain’s energy system was investigated under different flexibility options and uncertainty characterisations. Results indicate that electricity storage was the most effective measure to reduce operation cost. The impacts of uncertainty characterisation methods were significant only when system flexibility was insufficient.

Traditionally, state estimation is applied to transmission networks to improve security and redundancy of the measurement system. This paper describes the application of state estimation to distribution networks in order to extend the observability of the network. Key features of this application are that the network is active, minimal real measurements are available and that there is minimal communications infrastructure. this paper presents results ffrom a field trial whihc manages voltages in an 11kV distribution network with distributed generation. The paper highlights the problems associated with this application and presents some solutions.

High penetration of distributed generation is currently limited by passive operating methods of distribution networks. At distribution level, insufficient measurements are available to allow satisfactory control, so measurement is extended through the use of state estimation. Aspects that are described in this paper include optimal measurement location, the problems occurring with few measurements, and load models used for “pseudo-measurements”. Two control philosophies are described. The first, applicable to smaller network segments, is a local control, working with existing control devices such as transformer automatic voltage control (AVC) relays. The second philosophy is optimal control with constraints.

Laboratory simulation, relay testing and a site trial have been used to both validate and develop a new type of protection for small to medium sized wind farms (0-20 MW). The protection, implemented within a multifunction relay, is sufficiently sensitive to detect all realistic in-zone faults and correctly discriminates with the LV fuses that detect LV cable faults. Similarly, the site trial has confirmed that the source based protection is stable during out-of-zone faults and other types of network disturbances. (5 pages)

Embedded generation is being connected increasingly to distribution networks. For many new renewable energy schemes, this will involve the connection of induction generators to weak rural networks. It has been shown by studies, and subsequent operating experience, that relatively high ratios of wind farm capacity to network short circuit level can be accommodated successfully if conditions are favourable. However, voltage stability of the power system is likely to be an important limiting factor to the continued increase of the ratio of generation capacity to network short circuit level.

A new type of relay suitable for the protection of wind farms has been described. The relay, installed at the utility end of a power collection circuit, is designed to protect the collection circuit and the LV and MV windings of each wind turbine generator transformer. The relay has been tested on-site at the Cemmaes wind farm and in a laboratory using the EMTDC simulator and an OMICRON relay test set. Numerous tests have shown that the relay remains stable during out-of-zone faults or nonfault disturbances and is sufficiently sensitive to operate on all realistic in-zone faults. (6 pages)