Studies of dynamic interactions in hybrid ac-dc grid under different fault conditions using real time digital simulation

High Voltage Direct Current (HVDC) grid concept has recently been introduced as the next step to the point-to-point HVDC links to maximise the benefits of more power flow share, and reduce the impact of renewables intermittency. However, the integration of DC grid within existing AC systems is complex and very challenging. The compatibility between the two systems under different operating conditions needs to be fully understood. This will require sophisticated models and analysis of their behaviour using powerful tools. This paper presents a representative dynamic model of a hybrid AC-DC gird, and investigates the transient and dynamic interaction between the AC and DC grids under different fault conditions. The model represents a reduced AC UK power system interfaced to a detailed four-terminal DC grid. The developed mode is built in Real Time Digital Simulation (RTDS) using dual time steps simulation techniques

Feasibility of direct current street lighting & integrated electric vehicle charging points

In the context of the energy trilemma (the provision of sustainable, affordable, reliable energy) the application of Low Voltage Direct Current (LVDC) distribution offers several advantages over the incumbent AC distribution system. LVDC distribution can increase the power transfer capability of existing cable assets while reducing the converter complexities required to integrate distributed generators and modern electrical loads to the network. This paper evaluates the technical potential for LVDC street lighting and integrated electric vehicle charging points by considering existing cable specifications, protection schemes and overall system efficiency. The first author was supported by EPSRC Centre for Doctoral Training in Future Power Networks and Smart Grids (EP/L015471/1) with industry support from Rolls-Royce plc. No new data was collected or generated during the course of the research

Validation of fast and selective protection scheme for an LVDC distribution network

Low Voltage Direct Current (LVDC) distribution systems are one of the emerging technologies to recently attract attention for more efficient use of energy, and wider uptake of distributed renewables and energy storage. They do however present significant fault protection and safety challenges, which are not possible to address without using advanced protection techniques. Therefore, this paper considerably reviews these key challenges, and presents experimental results of prototyping an advanced protection scheme developed to help enable LVDC distribution networks for utility applications. The developed scheme is a DC current direction-based using multiple intelligent electronic devices (IEDs) relays in combination with controllable solid-state circuit breakers to detect and locate DC faults, and provide selective protection tripping within sub-millisecond timescales. A scaled laboratory demonstrator that emulates an LVDC distribution network is used as a test platform. It allows the characterisation of the transient behaviour for various fault conditions and locations. The developed protection algorithm is implemented in LabVIEW, and its performance against such fault conditions is tested within this environment

A co-simulation approach using powerfactory and matlab/simulink to enable validation of distributed control concepts within future power systems

In power network analysis it is increasingly desirable to implement controller and power systems models within different software environments. This stems from, among other things, an increasing influence of new and distrib-uted control functions within smart grids and a growing influence of market operations. The computation time re-sulting from use of multiple simulation environments can cause significant delays and constrain the number of scenarios considered. This paper introduces and com-pares several techniques for integrating external control system models into power systems models for time do-main simulations. In particular, a new technique is reported in this paper for PowerFactory-MATLAB/Simulink co-simulation interfaces, which offers a significant advantage over alternative methods in terms of the reduction in simulation runtimes and flexi-bility for the end user

Investigation of a decentralised control strategy for grid frequency support from DC microgrids

DC microgrids are capable of integrating and coordinating distributed energy resources into power systems, along with providing services to the wider system e.g. balancing, frequency support, demand response, etc. This paper investigates the capability of DC microgrids providing grid frequency support, for the GB Enhanced Frequency Response service. In this study, photovoltaic generation, energy storage, and load, form the DC microgrids. The control strategy is a decentralised scheme, based on conventional droop control for active power sharing and grid frequency support. Droop control is also used within the DC microgrid for power sharing amongst generation and load. Scenarios are conducted to evaluate the effectiveness of the control strategy and verified by MATLAB/Simulink simulations

Non-parametric identification techniques for intelligent pneumatic actuator

The aim of this paper is to present experimental, empirical and analytic identification techniques, known as non-parametric techniques. Poor dynamics and high nonlinearities are parts of the difficulties in the control of pneumatic actuator functions, which make the identification technique very challenging. Firstly, the step response experimental data is collected to obtain real-time force model of the intelligent pneumatic actuator (IPA). The IPA plant and Personal Computer (PC) communicate through Data Acquisition (DAQ) card over MATLAB software. The second method is approximating the process by curve reaction of a first-order plus delay process, and the third method uses the equivalent n order process with PTn model parameters. The obtained results have been compared with the previous study, achieved based on force system identification of IPA obtained by the (Auto-Regressive model with eXogenous) ARX model. The models developed using non-parameters identification techniques have good responses and their responses are close to the model identified using the ARX system identification model. The controller approved the success of the identification technique with good performance. This means the Non-Parametric techniques are strongly recommended, suitable, and feasible to use to analyze and design the force controller of IPA system. The techniques are thus very suitable to identify the real IPA plant and achieve widespread industrial acceptance

Improved voltage-based protection scheme for an LVDC distribution network interfaced by a solid state smart transformer

The increasing electrification of transport and heat will place increasing demand on low voltage (LV) networks with the potential to overload medium voltage (MV)/LV transformers and LV cables. Deployment of a solid-state transformer (SST) at MV/LV substations and using LV direct current (LVDC) distribution systems offer great potential to address such challenges. However, the SST deployment in addition to the introduction of LVDC will fundamentally change LV fault behaviour and protection requirements due to the limited short-circuit capabilities of such technologies. The SST will deliver limited fault currents, making current-based protection (widely used in LV networks) less reliable. Therefore, this study presents an advanced communication-less protection scheme which can effectively detect and locate DC faults even with reduced fault levels. The developed protection scheme overcomes the selectivity limitations in LVDC voltage-based protection solutions by using a combination of DC voltage magnitude, voltage concavity (sign of d2v/dt2) and the sign of the rate of change of current (di/dt) regardless of the current magnitudes. The credibility of the developed protection algorithm is tested against different fault scenarios applied on an active LVDC network model built in PSCAD/EMTDC. Noise signals have been included in the simulation to appraise the resilience of the developed scheme

Cyber-physical energy systems modeling, test specification, and co-simulation based testing

The gradual deployment of intelligent and coordinated devices in the electrical power system needs careful investigation of the interactions between the various domains involved. Especially due to the coupling between ICT and power systems a holistic approach for testing and validating is required. Taking existing (quasi-) standardised smart grid system and test specification methods as a starting point, we are developing a holistic testing and validation approach that allows a very flexible way of assessing the system level aspects by various types of experiments (including virtual, real, and mixed lab settings). This paper describes the formal holistic test case specification method and applies it to a particular co-simulation experimental setup. The various building blocks of such a simulation (i.e., FMI, mosaik, domain-specific simulation federates) are covered in more detail. The presented method addresses most modeling and specification challenges in cyber-physical energy systems and is extensible for future additions such as uncertainty quantification

An advanced protection scheme for enabling an LVDC last mile distribution network

Low voltage direct current (LVDC) distribution systems have the potential to support future realisation of smart grids and enabling of increased penetration of distributed renewables, electric vehicles, and heat pumps. They do however present significant protection challenges that existing schemes based on DC fuses and conventional electro-mechanical circuit breakers (EMCBs) cannot manage due to the nature of DC faults and slow device performance. Therefore, this paper presents an advanced protection scheme that addresses the outstanding challenges for protecting an LVDC last mile distribution network. The scheme takes advantage of advanced local measurements and communications that will be naturally integrated in smart grids, and the excellent level of controllability of solid state circuit breakers. It thus provides fast DC fault detection and interruption during DC transient periods in addition to achieving fault limitation and fast reliable restoration. The introductory part of the paper quantifies the potential benefits of LVDC last mile distribution networks, and discusses the potential LVDC architectures that best utilise the existing plant. Based on the new LVDC architectures, a typical UK LV network is energised using DC and modelled, and used as a case study for investigating the protection issues and evaluating the new protection scheme performance through simulation