Philip Ciufo

Microgrids of Commercial Buildings: Strategies to Manage Mode Transfer From Grid Connected to Islanded Mode

Microgrid systems located within commercial premises are becoming increasingly popular and their dynamic behavior is still uncharted territory in modern power networks. Improved understanding in design and operation is required for the electricity utility and building services design sectors. This paper evaluates the design requirements for a commercial building microgrid system to facilitate seamless mode transition considering an actual commercial building microgrid system. A dynamic simulation model of the proposed microgrid system is established (utilizing DIgSILENT Power Factory) to aid the development of planning and operational philosophy for the practical system. An economic operational criterion is developed for the microgrid to incorporate selective mode transition in different time intervals and demand scenarios. In addition, a multi-droop control strategy has been developed to mitigate voltage and frequency variations during mode transition. Different system conditions considering variability in load and generation are analyzed to examine the responses of associated microgrid network parameters (i.e., voltage and frequency) with the proposed mode transition strategy during planned and unplanned islanding conditions. It has been demonstrated that despite having a rigorous mode transition strategy, control of certain loads such as direct online (DOL) and variable-speed-drive (VSD) driven motor loads is vital for ensuring seamless mode-transition, in particular for unplanned islanding conditions.

Voltage Unbalance Emission Assessment in Radial Power Systems

Voltage unbalance (VU) emission assessment is an integral part in the VU management process where loads are allocated a portion of the unbalance absorption capacity of the power system. The International Electrotechnical Commission Report IEC/TR 61000-3-13:2008 prescribes a VU emission allocation methodology establishing the fact that the VU can arise at the point of common connection (PCC) due to both upstream network unbalance and load unbalance. Although this is the case for emission allocation, approaches for post connection emission assessment do not exist except for cases where the load is the only contributor to the VU at the PCC. Such assessment methods require separation of the post connection VU emission level into its constituent parts. In developing suitable methodologies for this purpose, the pre- and post-connection data requirements need to be given due consideration to ensure that such data can be easily established. This paper presents systematic, theoretical bases which can be used to assess the individual VU emission contributions made by the upstream source, asymmetrical line and the load for a radial power system. The methodology covers different load configurations including induction motors. Assessments obtained employing the theoretical bases on the study system were verified by using unbalanced load flow analysis in MATLAB and using DIgSILENT PowerFactory software.

Point of common coupling (PCC) voltage control of a grid-connected solar photovoltaic (PV) system

In future low voltage grids, with multiple inverter interfaced sources connected, voltage regulation may become a necessary task. The potential exists for inverter interfaced sources to be deployed to regulate the voltage at the point of common coupling (PCC) of each inverter interfaced sources. The PCC voltage regulation is attainable with inverter interfaced sources by dynamically controlling the amount of reactive power injected to the power distribution grid by individual systems. In the current research, a closed-loop controller is proposed to regulate the PCC voltage of a solar photovoltaic (PV) system that is connected to a single-phase power distribution feeder (with R to X ratio greater than 1). The plant model of the PCC voltage controller of the PV system is derived considering both reactance and resistance of the network to which the PV system is connected. Three different compensators are evaluated to identify a suitable compensator for the closed-loop PCC voltage controller to regulate the PCC voltage at a given reference voltage. Simulation studies and experimental verification confirm that the theoretical approach taken to derive the control plant model of the PCC voltage controller is accurate and the procedure that is followed to design the controller is robust. The control design procedures illustrated in the current research leads to a PCC voltage control system with acceptable dynamic and steady state performance.

Intelligent load management in Microgrids

The increased levels of distributed generator (DG) penetration and the customer demand for high levels of reliability have attributed to the formation of the Microgrid concept. The Microgrid concept contains a variety of technical challenges, including load management and anti-islanding protection discrimination strategies. This paper provides a novel scheme in which loads and DG are able to detect the conditions where the load of the island cannot be sufficiently supplied. In these instances, a load shedding algorithm systematically removes loads from the system until an island can be maintained within satisfactory operating limits utilising the local DG. The concept of an Intelligent Load Shedder (ILS) module is proposed in this paper. This module is connected in series with non-critical loads in order to detect the conditions where that non-essential load should be isolated from an island. This module must be capable of communicating with the static transfer switch (STS), which is the intelligent isolator associated with the island. The STS will also be capable of sending and receiving data with each DGs islanding protection device. The combined algorithmic control of the STS, ILS module and DG islanding protection device forms the Intelligent Load Management algorithm. This algorithm is capable of islanding protection and load shedding irrespective of the use of communications. The algorithms within this paper are simulated using MATLAB script. The results show that, on a theoretical level, the intelligent load management scheme described in this paper can be used to detect the conditions where an insufficient load is available using local parameters. Load shedding coordination is also shown to be possible with and without the use of communications between the STS, ILS module and DG islanding protection module.

Lifetime analysis of aluminum electrolytic capacitor subject to voltage fluctuations

This paper evaluates the changes in the ripple current for an electrolytic capacitor used in the dc-side of a single-phase rectifier circuit when subjected to input voltage fluctuations. The study has been undertaken in order to analyse the potential impact on capacitor lifetime. The key effect is that the capacitor ripple current, as a consequence of voltage fluctuations, increases dramatically and this phenomenon keeps deteriorating as the frequency of the voltage fluctuations increases. Simulations and experimental work confirm this phenomenon. Since the power loss and temperature rise are dependent on the capacitor equivalent series resistance (ESR) and ripple current components, an increase in ripple current under voltage fluctuation conditions is likely to accelerate this process, resulting in a reduced lifetime.

Voltage Unbalance Emission Assessment in Interconnected Power Systems

The International Electrotechnical Commission (IEC) Technical Report IEC/TR 61000-3-13:2008 considers voltage-unbalance (VU) emission assessment as a key aspect of the VU management of power systems. Compliance assessment of unbalanced installations at the postconnection stage is essential to ensure that the limits set by the IEC VU emission allocation methodology in the preconnection stage are met. Although VU is known to be caused by load asymmetries and inherent network asymmetries, locating all VU emission sources is not a straightforward process, especially in a network with interconnections. These assessment methodologies should ensure that the contributions from various sources of unbalance to the total VU emission are determined using data which are not overly demanding. This paper presents deterministic methodologies which can be used to assess constituent components of the postconnection VU level at the point of evaluation in an interconnected network utilizing postconnection voltage/current measurements and known system parameters. The theoretical bases are developed to cover different load types including induction motors. Emission assessment outcomes of different study systems obtained by employing the proposed methodologies are verified usingInternational Electrotechnical Commission (IEC) Technical Report IEC/TR 61000-3-13:2008 unbalanced load-flow analysis.

Bitstream Binding Language – Mapping XML Multimedia Containers into Streams

Bitstream binding language (BBL) provides an abstraction layer between XML multimedia containers and the way their resources and metadata are published in a bitstream. It allows multiple bindings from a single source document to facilitate interoperability and applicability of the multimedia content to a wide range of terminals and users. BBL introduces a number of features not found in other XML fragmentation techniques-such as support for random-access-and provides a highly general framework for combining metadata with multimedia content in a transport format. This paper presents an overview of BBL and provides details of a Java implementation using the MPEG-21 multimedia framework as a sample application

Balancing Energy in the Smart Grid Using Distributed Value Function (DVF)

As the penetration of renewable energy resources increases in distribution networks, so does the need to manage these resources in an effective manner. Since these resources are installed to displace carbon-based generation and to provide an income stream to the resource owner, it is important that both installation objectives (as a minimum) can be met. With all of these renewable energy resources available, the opportunity also exists to assist with the energy management of this resource-rich distribution network. However, the renewable energy resources do not produce power in a deterministic manner. The available production depends on the time of day and many other environmental factors. Accordingly, a system that is able to program and coordinate the production and storage of power in a distribution network would be of benefit to the network operator. This paper presents a multiagent system (MAS) that is responsible for the management of renewable energy resources and power storage systems connected to the distribution network of a zone substation. The MAS manages the orderly connection and disconnection of resources using a plug and play algorithm in order to minimize disturbances to the supply-and-demand balance within the distribution network. The proposed MAS design is validated using a network based on the IEEE 34-bus test feeder. The results obtained through computer simulation show that with the MAS, it is possible to manage the power resources so that there is minimal power drawn from the upstream network during periods of high demand.

Over-voltage mitigation within distribution networks with a high renewable distributed generation penetration

The rapid growth of grid-connected distributed generation has increased the likelihood of over-voltage occurrences in distribution networks. In recent times, much research has taken place in order to develop a control strategy to mitigate the voltage rise problem. However, most of the published strategies require re-tuning when additional resources are connected, or have a strong dependence on network parameters, such as fault level. This paper proposes a novel over-voltage mitigation scheme that has many advantages not observed in literature. Firstly, the control scheme can integrate with an existing feeder in a plug-and-play fashion. No prior analysis is necessary to configure the control parameters; all required information is measured locally. Secondly, the control scheme is a simple extension upon constant power control which is common in most grid-connected inverter interfaces. Finally, the proposed over-voltage mitigation scheme enforces a fair and equitable power flow allocation. The scheme contains a predefined point of convergence for any voltage magnitude measured at the point of common coupling. Many control schemes operate in a perturb and observe manner which can inadvertently allow certain DG units to export a disproportionate amount of power with respect to other DG units. This paper also details a methodology for analysing the cost effectiveness of any given DG configuration utilising the proposed over-voltage mitigation scheme. The analysis is useful for determining whether a network infrastructure upgrade may be necessary as power curtailment becomes more prevalent within a distribution network.

Induction motors subject to regular voltage fluctuations: Stator and rotor current analysis from a heating perspective

Induction motors are widely applied in industry. Their performance strongly depends on relative power supply quality, such as voltage sags, harmonics, voltage unbalance and voltage fluctuations. This paper investigates the impact of voltage fluctuations on induction motor performance, particularly the stator and rotor current variation characteristics. An induction motor steady state analysis method is proposed when the motor is subjected to regular voltage fluctuations. The dynamic model of an induction motor is used in order to represent the induction motor instantaneous response. Furthermore, the correlative simulation and experiment work are presented in order to illustrate the impact of voltage fluctuations. The stator and rotor RMS current magnitudes of the motor increase dramatically and this effect keeps deteriorating with modulation frequency and voltage change increases. The increase in RMS current will increase losses, resulting in motor winding temperature rise, subsequently accelerating the ageing process.