Modeling and simulation enabled UAV electrical power system design

With the diversity of mission capability and the associated requirement for more advanced technologies, designing modern unmanned aerial vehicle (UAV) systems is an especially challenging task. In particular, the increasing reliance on the electrical power system for delivering key aircraft functions, both electrical and mechanical, requires that a systems-approach be employed in their development. A key factor in this process is the use of modeling and simulation to inform upon critical design choices made. However, effective systems-level simulation of complex UAV power systems presents many challenges, which must be addressed to maximize the value of such methods. This paper presents the initial stages of a power system design process for a medium altitude long endurance (MALE) UAV focusing particularly on the development of three full candidate architecture models and associated technologies. The unique challenges faced in developing such a suite of models and their ultimate role in the design process is explored, with case studies presented to reinforce key points. The role of the developed models in supporting the design process is then discussed

Multi-sample differential protection scheme in DC microgrids

This paper proposes a novel solution to the issue of protection instability caused by time synchronization error in high-speed differential protection schemes for DC microgrids. DC microgrids provide a more efficient platform to integrate fast-growing renewable energy sources, energy storage systems, and electronic loads. However, the integration of distributed generators (DG) may result in variable fault current magnitude and direction during fault conditions, potentially causing mis-coordination of conventional time graded overcurrent relays. One identified solution to this issue utilizes high-speed differential protection schemes to maintain effective selectivity in DG-dominated DC microgrids. However, as DC short-circuit fault currents are highly transient, microseconds of synchronization error in the measured line currents may cause protection stability issues, whereby mal-operation of relays may occur as a result of faults external to the protected zone. This paper investigates the impact of time synchronization errors for high-speed differential protection in DC distribution systems. It then proposes a multi-sample differential (MSD) scheme that performs multiple differential comparisons over a sampling window to ensure the stability of high-speed differential protection schemes for external faults whilst maintaining sensitivity to internal faults

Impact of key design constraints on fault management strategies for distributed electrical propulsion aircraft

Electrically driven distributed propulsion has been presented as a possible solution to reduce aircraft noise and emissions, despite increasing global levels of air travel. In order to realise electrical propulsion, novel aircraft electrical systems are required. Since the electrical system must maintain security of power supply to the motors during flight, the protection devices employed on an electrical propulsion aircraft will form a crucial part of system design. However, electrical protection for complex aircraft electrical systems poses a number of challenges, particularly with regard to the weight, volume and efficiency constraints specific to aerospace applications. Furthermore, electrical systems will need to operate at higher power levels and incorporate new technologies, many of which are unproven at altitude and in the harsh aircraft environment. Therefore, today’s commercially available aerospace protection technologies are likely to require significant development before they can be considered as part of a fault management strategy for a next generation aircraft. By mapping the protection device trade space based on published literature to date, the discrepancy between the current status of protection devices and the target specifications can be identified for a given time frame. This paper will describe a process of electrical network design that is driven by the protection system requirements, incorporates key technology constraints and analyses the protection device trade space to derive feasible fault management strategies

Evaluation of the impact of high bandwidth energy storage systems on DC protection

The integration of high bandwidth energy storage systems (ESS) in compact DC electrical power systems can increase the operational capability and overall flexibility of the network. However, the impact of ESSs on the performance of existing DC protection systems is not well understood. This paper identifies the key characteristics of the ESS that determine the extent of the protection blinding effects on slower acting generator systems on the network. It shows that higher fault impedances beyond that of an evaluated critical level will dampen the response of slower acting generator systems, decreasing the speed of corresponding overcurrent protection operation. The paper demonstrates the limitations of existing protection solutions and identifies more suitable protection approaches to remove/minimize the effects of protection blinding

Metrology requirements of state-of-the-art protection schemes for DC microgrids

Environmental incentives to combat climate change are providing the motivation to improve the energy efficiency of power distribution systems and integrate state-of-the-art renewable technologies. Examples include wind/PV resources, energy storage systems and electric vehicles integrated via efficient power electronic converters (PEC). Subsequently, DC microgrids (MGs) and distribution systems are receiving considerable attention in the literature because they offer a simple, yet flexible, interface between these modern resources and consumers. However, many technical challenges relating to the design and standardization of DC protection devices still exist that must be overcome prior to widespread adoption. For example, many protection schemes tailored for DC MGs have been proposed but few of them have considered the metrology requirements for practical implementation. This paper will first review the key features of DC-side fault transients simulated on a DC MG model in MATLAB/Simulink, and analyse the disruptive impact on PEC components. Secondly, a review of newly published DC protection schemes is performed. These protection schemes are classified by their fundamental operating principles and mathematically-derived metrology requirements are given

DC arc fault detection methods in MEA distribution systems

Direct current (DC) for primary power distribution is a promising solution that is being explored by aircraft system integrators for MEA applications to enable the paralleling of non-synchronized engine off-take generators, and to enable the reduction of energy conversion stages required to supply electronically actuated loads. However, a significant challenge in the use of DC systems is the reliable detection of arc faults. Arcing presents a significant fire risk to aircraft and their presence can result in critical system damage and potentially fatal conditions. Series arc faults in DC systems are particularly challenging to detect as the associated reduction in system current eliminates the use of conventional overcurrent and current differential methods for fault detection. This paper provides an overview of series arc faults in DC systems and presents both simulation and hardware results to illustrate key trends, characteristics and discriminating features. It also presents a comprehensive review of arc fault detection and diagnosis techniques that have been proposed for a wide range of aerospace and other applications. The paper concludes with a discussion on the unique challenges and opportunities for the application of both deterministic and probabilistic methods in MEA systems

Modulated low fault-energy protection scheme for DC smart grids

DC smart grids enabled by the integration of advanced power electronic converters (PEC) can ease the integration and control of distributed renewable energy resources, electric vehicles and energy storage systems. However, these highly flexible power systems introduce many challenges when considering the design of reliable, plug-and-play protection that does not rely on dedicated communications infrastructure for device coordination. One particularly difficult challenge is the management of DC-side filter capacitor discharge during short-circuit faults where the large peak fault-current produced can permanently damage exposed semiconductor components within the converter. One solution is to ensure that the trip-time of DC protection devices is sufficiently rapid (sub-millisecond) to guarantee that fault-current is blocked prior to reaching destructive magnitudes. However, such high-speed protection devices do not offer much margin for effective selectivity with downstream devices due to the narrow time window of operation. Accordingly, this paper proposes a non-unit protection scheme for future large-scale DC smart grid applications that increases this time-window of operation to enable improved selectivity whilst retaining a lower level of energy dissipated in the fault. Reliable protection coordination is demonstrated on a DC radial network and is realized using conventional millisecond trip-time devices, and a single solid-state microsecond trip-time device

On the protection of compact DC power systems with high-power energy storage

High-power energy storage systems (ESS) are being considered for future aerospace platforms and other compact DC power system applications to improve the overall transient performance of electrical power distribution systems. These sources are being integrated with advanced bidirectional power electronic converter interfaces with high bandwidth control systems and current limiting functionality. To date, the literature has primarily focused on the control and behaviour of high-power ESS during normal operating conditions with an emphasis on the systems level benefits they offer. Little consideration has been given to their response during network fault conditions.Through simulation and hardware experimentation, this thesis demonstrates that an ESS, by design, can contribute significant levels of current to a fault as it attempts to sustain the network voltage. This behaviour inadvertently reduces the fault current contribution from the primary source of power on the network, reducing the effectiveness of associated protection devices (protection blinding).The impact of several key DC power system design and operation parameters on the ESS fault response is quantified and a new critical fault impedance term, beyond which protection blinding can be expected to occur, is introduced. Building upon this new knowledge, enhancements to typical compact DC power system protection schemes which more effectively account for the presence of ESS are proposed and evaluated.Differential protection schemes are shown to eliminate protection blinding whilst offering the greatest flexibility in increasing protection speed and fault discrimination, and maximising ESS availability. Adaptive protection schemes are shown to be a reliable backup option where a consistent protection system response can be obtained despite the potentially intermittent nature of the ESS fault current contribution. A novel control strategy that actively modifies the fault response of the ESS to facilitate the use of conventional overcurrent schemes is also proposed and demonstrated for applications where communications-based protection is unfavourable. The thesis concludes by proposing a framework to guide protection engineers in the selection of appropriate protection and control strategies when considering the integration of high-power ESS within compact DC power systems.High-power energy storage systems (ESS) are being considered for future aerospace platforms and other compact DC power system applications to improve the overall transient performance of electrical power distribution systems. These sources are being integrated with advanced bidirectional power electronic converter interfaces with high bandwidth control systems and current limiting functionality. To date, the literature has primarily focused on the control and behaviour of high-power ESS during normal operating conditions with an emphasis on the systems level benefits they offer. Little consideration has been given to their response during network fault conditions.Through simulation and hardware experimentation, this thesis demonstrates that an ESS, by design, can contribute significant levels of current to a fault as it attempts to sustain the network voltage. This behaviour inadvertently reduces the fault current contribution from the primary source of power on the network, reducing the effectiveness of associated protection devices (protection blinding).The impact of several key DC power system design and operation parameters on the ESS fault response is quantified and a new critical fault impedance term, beyond which protection blinding can be expected to occur, is introduced. Building upon this new knowledge, enhancements to typical compact DC power system protection schemes which more effectively account for the presence of ESS are proposed and evaluated.Differential protection schemes are shown to eliminate protection blinding whilst offering the greatest flexibility in increasing protection speed and fault discrimination, and maximising ESS availability. Adaptive protection schemes are shown to be a reliable backup option where a consistent protection system response can be obtained despite the potentially intermittent nature of the ESS fault current contribution. A novel control strategy that actively modifies the fault response of the ESS to facilitate the use of conventional overcurrent schemes is also proposed and demonstrated for applications where communications-based protection is unfavourable. The thesis concludes by proposing a framework to guide protection engineers in the selection of appropriate protection and control strategies when considering the integration of high-power ESS within compact DC power systems

Optimized network planning of mini-grids for the rural electrification of developing countries

1.2 billion people, predominantly living in remote rural regions in countries of the Global South, currently live without access to any modern source of energy. Options for electrification of these communities include extending existing national grid infrastructure, deploying mini-grids, and installing standalone home systems (SHS). Deriving the most cost effective means of delivering energy to these consumers is a complex, multidimensional problem that normally requires determination on a case-by-case basis. However, optimization of the network planning may help to maximize the socio-economic return of the installed energy system. This paper presents an optimization process that minimizes the installation cost of a mix of generation sources for a rural mini-grid using a multi-objective particle swarm optimization (MOPSO) technique. Minimizing the cost of distribution layout is first formulated as a capacitated minimum spanning tree (CMST) problem and solved using the Esau-Williams method. Multiple cable sizes and source locations are then added to create a multi-level capacitated minimum spanning tree (MLCMST) problem, solved via a Genetic Algorithm (GA) employing Prim-Pred encoding. The method is applied to a case study village in India

Practical computation of di/dt for high-speed protection of DC microgrids

DC microgrids have the potential to radically disrupt the distribution system market due to the benefits offered in easing the integration and control of distributed renewable energy resources and energy storage systems. However, the nonzero-crossing fault current profiles associated with short-circuited DC systems present a major challenge for protection. Isolation of faulted networks prior to the peak-current discharge of DC side capacitors may address this challenge if rapid fault detection speeds (shorter than 2ms) can be achieved. Accordingly, novel methods of utilizing the rate-of-change-of-current (di/dt) have been proposed in the literature to realize new, high-speed distance protection strategies. This paper proposes two practical methods for optimizing the numerical computation of di/dt of fault current transients and evaluates the performance of each within a MATLAB/Simulink model of a DC microgrid with artificially injected measurement noise