SmartGrid: Future networks for New Zealand power systems incorporating distributed generation

The concept of intelligent electricity grids, which primarily involves the integration of new information and communication technologies with power transmission lines and distribution cables, is being actively explored in the European Union and the United States. Both developments share common technological developmental goals but also differ distinctly towards the role of distributed generation for their future electrical energy security. This paper looks at options that could find relevance to New Zealand (NZ), in the context of its aspiration of achieving 90% renewable energy electricity generation portfolio by 2025. It also identifies developments in technical standardization and industry investments that facilitate a pathway towards an intelligent or smart grid development for NZ. Some areas where policy can support research in NZ being a “fast adapter” to future grid development are also listed.This paper will help policy makers quickly review developments surrounding SmartGrid and also identify its potential to support NZ Energy Strategy in the electricity infrastructure. This paper will also help researchers and power system stakeholders for identifying international standardization, projects and potential partners in the area of future grid technologies.     

Photovoltaic solar system connected to the electric power grid operating as active power generator and reactive power compensator

In the case of photovoltaic (PV) systems acting as distributed generation (DG) systems, the DC energy that is produced is fed to the grid through the power-conditioning unit (inverter). The majority of contemporary inverters used in DG systems are current source inverters (CSI) operating at unity power factor. If, however, we assume that voltage source inverters (VSI) can replace CSIs, we can generate reactive power proportionally to the remaining unused capacity at any given time. According to the theory of instantaneous power, the inverter reactive power can be regulated by changing the amplitude of its output voltage. In addition, the inverter active power can be adjusted by modifying the phase angle of its output voltage. Based on such theory, both the active power supply and the reactive power compensation (RPC) can be carried out simultaneously. When the insolation is weak or the PV modules are inoperative at night, the RPC feature of a PV system can still be used to improve the inverter utilisation factor. Some MATLAB simulation results are included here to show the feasibility of the method.     

A multi-agent system for energy management of distributed power sources

The field of energy management is an area increasingly studied. However, most solutions are based on centralized systems and barely fulfil criterion like fault tolerance or adaptability. Also, these systems are often difficult to design because of the “top–down” approach used: the designer generally knows how each component has to respond separately, but a centralized management system focuses his attention solely on the overall reaction of the system. That is why a distributed management solution based on the paradigm of Multi-Agent Systems (MASs) is proposed in this paper. In addition to a more natural conception, based on a “bottom–up” approach, this solution ensures better system reliability. After reviewing the previous works, an application of MAS to power management in a hybrid power source is presented. Then, the system is tested using a simulation model. The results show that this approach is perfectly valid and can respond to most problems of centralized energy management systems (EMSs).     

Battery-integrated boost converter utilizing distributed MPPT configuration for photovoltaic systems

In this paper, a battery-integrated boost converter utilizing the distributed maximum power point tracking (DMPPT) configuration for a photovoltaic (PV) system is studied. Each PV module has its own battery and DC/DC converter. Due to the proposed topology and use of battery, the MPPT function is not affected by the load demand and input power from PV. Application of the proposed converter to DMPPT configuration can save the voltage amplification stage and maintain PV voltage during partial shading. Steady-state analysis of the converter to determine the power flow equations is presented. Comparison with the series-connected conventional boost converter is reported in this paper. Simulation and experiment results of a laboratory prototype are presented to verify the effectiveness of the proposed approach. System design considerations are also discussed.     

Optimization of distributed hybrid power systems employing multiple fuel-cell vehicles

This paper investigates the benefits of distributed hybrid power systems employing multiple fuel-cell vehicles. In earlier work, our optimization of hybrid power systems showed that a single fuel cell acting as backup power to guarantee energy sustainability operates for less than 3% of the time but incurs more than 16% of the system costs. Therefore, the system cost could be reduced when applying a fuel-cell vehicle to dynamically support twelve power stations. Here, we extend this idea by employing multiple fuel-cell vehicles to support more power stations. We develop a power management strategy and optimize the management parameters by the genetic algorithm. The results show a reduction of more than 21% by applying multiple fuel-cell vehicles in the distributed systems. Experiments also confirm the feasibility of using multiple fuel-cell vehicles. Based on the results, the proposed systems are deemed effective for reducing system costs while maintaining system sustainability.     

A review of power electronics interfaces for distributed energy systems towards achieving low-cost modular design

Due to increased attention towards clean and sustainable energy, distributed energy (DE) systems are gaining popularity all over the world. Power electronics are an integral part of these energy systems being able to convert generated electricity into consumer usable and utility compatible forms. But the addition of power electronics adds costs to the DE capital investments along with some reliability issues. Therefore, widespread use of distributed energy requires power electronics topologies that are less expensive and more dependable. Use of modular power electronics is a way to address these issues. Adoption of functional building blocks that can be used for multiple applications results in high volume production and reduced engineering effort, design testing, onsite installation and maintenance work for specific customer applications. In this paper, different power electronics topologies are reviewed that are typically used with distributed energy systems. The integrated power electronics module (IPEM) based back-to-back converter topologies are found to be most suitable interface that can operate with different DE systems with small or no modifications. Also the requirements for a hierarchical control structure with standardized power and communication interfaces are addressed in the paper along with some discussion on future design possibilities for the IPEM-based power electronics topologies. It is expected that modular and flexible power electronics and standardized controls and interfaces, will provide commonality in hardware and software for the power electronics interfaces, thus will enable their volume production and decrease their cost share in distributed energy applications.     

VSC-based direct torque and reactive power control of doubly fed induction generator

This paper proposes a novel direct torque and reactive power control (DTC) for grid-connected doubly fed induction generators (DFIGs) in the wind power generation applications. The proposed DTC strategy employs a variable structure control (VSC) scheme to calculate the required rotor control voltage directly and to eliminate the instantaneous errors of active and reactive powers without involving any synchronous coordinate transformations, which essentially enhances the transient performance. Constant switching frequency is achieved as well by using space vector modulation (SVM), which eases the designs of power converter and ac harmonic filters. Simulated results on a 2 MW grid-connected DFIG system are presented and compared with those of the classic voltage-oriented vector control (VC) and traditional look-up-table (LUT) direct power control (DPC). The proposed VSC DTC maintains enhanced transient performance similar to the LUT DPC and keeps the steady-state harmonic spectra at the identical level as the VC strategy when the network is strictly balanced. Besides, the VSC DTC strategy is capable of fully eliminating the double-frequency pulsations in both the electromagnetic torque and the stator reactive power during network voltage unbalance.     

Techno-economics of small wind electric generator projects for decentralized power supply in India

A techno-economic evaluation of small wind electric generator (SWEG) projects for providing decentralized power supply in remote locations in India is presented. SWEG projects that have either been implemented or are under implementation have been considered. The capital costs of the SWEG projects and sub-systems have been analysed. Levelised unit cost of electricity (LUCE) has been estimated for 19 select places located in different geographical regions of the country. The LUCE is found to vary in the range of Rs. 4.67–83.02/kWh (US$¹ 0.10–1.86/kWh) for wind electric generator projects in the capacity range 3.2–50 kW with annual mean wind speed variation in the range 5–10 m/s. Issues relating to their environmental impact(s), barriers to diffusion and institutional mechanism(s) to implement such projects have also been discussed.     

Fuel-cell sharing for a distributed hybrid power system

This paper investigates the benefits of sharing a proton exchange membrane fuel cell (PEMFC) in a distributed hybrid power system. The PEMFC is usually used as backup power in stationary hybrid power systems; however, in that scenario, it might be working only 2% of the time while incurring 20% of the system expenses. Therefore, this paper examines the potential of sharing a PEMFC among multiple power systems. We develop a distributed hybrid power system that comprises several immovable power stations and a fuel-cell vehicle (FCV). Each power station is equipped with solar panels and batteries, while the FCV contains a PEMFC module and can move among the stations to provide sustainable power as needed. We propose power management strategies and show that the total system costs can be significantly reduced by 10.83% and 17.89% when sharing one FCV between three and twelve power stations, respectively. We also design experiments to demonstrate the feasibility of the proposed distributed hybrid power system. In the future, the developed model can be extended to provide further cost reductions by optimizing distributed hybrid power systems with multiple FCVs.     

A review of power converter topologies for wind generators

Wind energy conversion systems have become a focal point in the research of renewable energy sources. This is in no small part due to the rapid advances in the size of wind generators as well as the development of power electronics and their applicability in wind energy extraction. This paper provides a comprehensive review of past and present converter topologies applicable to permanent magnet generators, induction generators, synchronous generators and doubly fed induction generators. The many different generator–converter combinations are compared on the basis of topology, cost, efficiency, power consumption and control complexity. The features of each generator–converter configuration are considered in the context of wind turbine systems.     

A new fuzzy adaptive particle swarm optimization for daily Volt/Var control in distribution networks considering distributed generators

This paper presents a novel approach for daily Volt/Var control in distribution systems using Distributed Generation (DG) units. The impact of DG units on Volt/Var control is significant in a distribution network with radial configuration and small X/R ratio. In this paper, a price-based approach is adopted to determine the optimum active and reactive power dispatch for the DG units, the reactive power contribution of the capacitor banks, and the tap settings of the transformers in a day in advance. A fuzzy adaptive particle swarm optimization (FAPSO) method is used to solve the daily Volt/Var control which is a non-linear mixed-integer problem. A mathematical expression of the proposed method and its effectiveness using simulation results are provided.     

Reduced models of doubly fed induction generator system for wind turbine simulations

This article compares three reduced models with a detailed model of a doubly fed induction generator system for wind turbine applications. The comparisons are based on simulations only. The main idea is to provide reduced generator models which are appropriate to simulate normal wind turbine operation in aeroelastic wind turbine models, e.g. for control system design or structural design of the wind turbine. The electrical behaviour such as grid influence will therefore not be considered. The work presented in this article shows that with an ideal, undisturbed grid the dynamics of the doubly fed induction generator system is very well represented by the dynamics due to the generator inertia and the generator control system, whereas the electromagnetic characteristics of the generator can be represented by the steady state relations. The parameters for the proposed models are derived from parameters typically available from the generator data sheet and from the controller settings. Thus the models are simple to apply in any case where the generator data sheet is available. Copyright © 2005 John Wiley & Sons, Ltd.     

Pumped storage systems introduction in isolated power production systems

The present paper investigates the introduction of pumped storage systems (PSS) in isolated power production systems with high thermoelectric production and wind energy rejection.The introduced PSS aims at:

• the maximisation of the wind energy penetration and

• the minimisation of the energy production cost.

In former studies, the introduction of PSS in power production systems aims at the power demand peaks saving. In the present study, the PSS storage-production procedure is accomplished without any predefined schedule. Energy is stored whenever:

• wind energy is rejected and

• the thermal generators that burn cheap heavy fuel oil do not operate at their nominal powers.

Furthermore, the production of the thermal generators that burn expensive diesel oil is substituted during the power demand peak hours.Two case studies for Crete and Rhodes are accomplished. An iterative procedure is performed, in order to calculate the optimum pumps and hydro turbines nominal powers in both islands. The optimisation criterion is the energy production specific cost minimisation.The PSS introduction in Crete yields to almost 10% annual electricity production cost reduction. The annual wind energy rejection is nullified. The investment payback period may be less than 5 years.The PSS introduction in Rhodes leads to a 1.85% annual electricity production cost reduction. The PSS project does not exhibit attractive economic indexes.Conclusively, isolated power production systems with energy production specific cost higher than approximately 0.05€/kW h seem to be appropriate for PSS introduction, following the operation algorithm of the present paper.     

Comparative analysis of different grid-independent hybrid power generation systems for a residential load

Demand of electricity is rising all over the world, both in developing and developed countries due to escalation in world population and economic growth. The exploitation of renewable energy is imperative to mitigate energy crisis and to avoid the environmental downfall. The stochastic nature of many renewable energy sources sets techno-economic and functional limitations in their application for covering most types of energy needs. These limitations can be surmounted if a renewable and a conventional energy source are combined to formulate a hybrid generation power system.This paper examines the techno-economic feasibility of four hybrid power generation systems applied to cover the demand of a typical off-grid residence for a 20 years period. Each one of these hybrid power solutions should involve at least one renewable energy source technology and be able to cover all load needs. Four applications are investigated for each hybrid system, accounting to different geographical areas in Greece with diverse solar and aeolic profile. A comparative analysis is followed to set off the optimal solution based on a minimal total cost criterion.     

Nonlinear control of the doubly-fed induction generator in wind power systems

This paper describes the models of a wind power system, such as the turbine, generator, power electronics converters and controllers, with the aim to control the generation of wind power in order to maximize the generated power with the lowest possible impact in the grid voltage and frequency during normal operation and under the occurrence of faults. The presented work considers a wind power system equipped with the doubly-fed induction generator and a vector-controlled converter connected between the rotor and the grid. The paper presents comparative results between proportional-integral controllers and neural networks based controllers, showing that better dynamic characteristics can be obtained using neural networks based controllers.     

Integration of renewable power systems in an Antarctic Research Station

The paper describes the design process of a photovoltaic (PV)-wind power system to be installed in the very challenging ambient conditions of the French-Italian Antarctic Base. Concordia Base has been built with the collaboration of Italian consortium PRNA, French Polar Institute IPEV and European Space Agency ESA. It is one of the three bases not located on the coast and is open all the year. The electrical load of the base, presently supplied by three diesel generators, has been previously characterised measuring the relevant quantities during a period of one year. During the same year an experimental campaign has been conducted to collect the necessary solar irradiance and wind data of the site. Models of the PV panels and of the wind turbine, previously set up and validated, have been used to simulate the plant behaviour and to estimate the possible contribution of renewable energies to the Concordia Antarctic Base supply in the different seasons. Finally, some economical aspects are discussed and the payback period is calculated.     

Neuro-fuzzy control for autonomous wind–diesel systems using biomass

This paper deals with the development of a neuro-fuzzy controller for a wind–diesel system composed of a stall regulated wind turbine with an induction generator connected to an ac bus-bar in parallel with a diesel generator set having a synchronous generator. A gasifier is capable of converting tons of wood chips per day into a gaseous fuel that is fed into a diesel engine. The controller inputs are the engine speed error and its derivative for the governor part of the controller, and the voltage error and its derivative for the automatic voltage regulator. These are readily measurable quantities leading to a simple controller which can be easily implemented. It is shown that by tuning the fuzzy logic controllers, optimal time domain performance of the autonomous wind–diesel system can be achieved in a wide range of operating conditions compared to fixed-parameter fuzzy logic controllers and PID controllers.     

Study on synchronization phenomena of fixed speed wind turbines with an induction generator in a wind farm

Synchronization of rotor angles of fixed speed wind energy converters with an induction generator has been reported in several wind farms. In terms of impacts on a power system, the synchronization could worsen the voltage flicker problems. Although many articles have examined this synchronization, there remains considerable room in the physical interpretation of the phenomena. This article discusses the physical context of the synchronization phenomena of wind turbines in a wind farm. After analysing interaction among electric power output, voltage, generator slip and rotor angle, differential equations describing the phenomena are presented. The synchronization phenomena are examined through a study on equilibria of the equations and their stability. Fundamental characteristics of the phenomena such as pull‐in force, speed of synchronization and the impact of discrepancies among generator characteristics are discussed for two‐ and three‐machine systems. The results show that synchronization tends to occur in wind energy converters with large 3p components and a low power factor, which are interconnected into a grid with large short‐circuit impedance of high X/R ratio. Copyright © 2005 John Wiley & Sons, Ltd.     

Design requirements for medium-sized wind turbines for remote and hybrid power systems

This paper provides an overview of the design requirements for medium-sized wind turbines intended for use in a remote hybrid power system. The recommendations are based on first-hand experience acquired at the University of Massachusetts through the installation, operation, and upgrade of a 250-kW turbine on a mountain top with difficult access in Western Massachusetts. Experience with the operation of this turbine and the design of its control system, together with a long history in the design and analysis of hybrid power systems, has made it possible to extend the work in Western Massachusetts to remote or hybrid power systems in general. The University test site has many attributes of more remote sites and the overall wind turbine installation is typical of one that could power a hybrid wind system. For example, access to the site is limited due to steep terrain, snow, and environmental restrictions. Also, the power lines feeding the turbine exhibit voltage sags and phase imbalance, especially during start-up. This paper is based on the experience gained from the operation of this wind turbine and assesses the requirements for the design and operation of medium to large wind turbines in remote locations. The work summarizes lessons learned relative to: (1) sensors, communication, and control capabilities; (2) grid connection issues; and (3) weather-related problems. The final section of the paper focuses on design requirements to ensure successful installation and the completion of maintenance and repairs at remote sites.     

Impact of distributed and independent power generation on greenhouse gas emissions: Sri Lanka

Sri Lanka has a hydropower dominated power system with approximately two thirds of its generation capacity based on large hydro plants. The remaining one third are based on oil fired thermal generation with varying technologies, such as oil steam, Diesel, gas turbines and combined cycle plants. A significant portion of this capacity is in operation as independent power plants (IPPs). In addition to these, Sri Lanka presently has about 40 MWs of mini-hydro plants, which are distributed in the highlands and their surrounding districts, mainly connected to the primary distribution system. Further, there are a few attempts to build fuel wood fired power plants of small capacities and connect them to the grid in various parts of the country.

The study presented in this paper investigates the impact of these new developments in the power sector on the overall emissions and the greenhouse gas (GHG) emissions in particular. It examines the resulting changes to the emissions and costs in the event of developing the proposed coal power plant as an IPP under different investment and operational conditions. The paper also examines the impact on emissions with 80 MWs of distributed power in different capacities of wind, mini-hydro and wood fired power plants.

It is concluded that grid connected, distributed power generation (DPG) reduces emissions, with only a marginal increase in overall costs, due to the reduction in transmission and distribution network losses that result from the distributed nature of generation. These reductions can be enhanced by opting for renewable energy based DPGs, as the case presented in the paper, and coupling them with demand side management measures. It is also concluded that there is no impact on overall emissions by the base load IPPs unless they are allowed to change over to different fuel types and technologies.