Real Power Network Research Articles

Overgrid: A Fully Distributed Demand Response Architecture Based on Overlay Networks

In this paper, we present Overgrid , a fully distributed peer-to-peer (P2P) architecture designed to automatically control and implement distributed demand response (DR) schemes in a community of smart buildings with energy generation and storage capabilities. As overlay networks in communications establish logical links between peers regardless of the physical topology of the network, the Overgrid is able to apply some power balance criteria to its system of buildings, as they belong to a virtual microgrid, regardless of their physical location. We exploit an innovative distributed algorithm, called flow updating, for monitoring the power consumption of the buildings and the number of nodes in the network, proving its applicability in an Overgrid scenario with realistic power profiles and networks of up to 10 000 buildings. To quantify the energy balance capability of Overgrid, we first study the energy characteristics of several types of buildings in our university campus and in an industrial site to accurately provide some reference buildings models. Then, we classify the amount of “flexible” energy consumption, i.e., the quota that could be potentially exploited for DR programs. Finally, we validate Overgrid emulating a real P2P network of smart buildings behaving according to our reference models. The experimental results prove the feasibility of our approach.

Multi-criteria selection of offshore wind farms: Case study for the Baltic States

This paper presents a multi-criteria selection approach for offshore wind sites assessment. The proposed site selection framework takes into consideration the electricity network’s operating security aspects, economic investment, operation costs and capacity performances relative to each potential site. The selection decision is made through Analytic Hierarchy Process (AHP), with an inherited flexibility that aims to allow end users to adjust the expected benefits accordingly to their respective and global priorities. The proposed site selection framework is implemented as an interactive case study for three Baltic States in the 2020 time horizon, based on real data and exhaustive power network models, taking into consideration the foreseen upgrades and network reinforcements. For each country the optimal offshore wind sites are assessed under multiple weight contribution scenarios, reflecting the characteristics of market design, regulatory aspects or renewable integration targets.

Model free graphical index for transient stability limit based on on‐line single machine equivalent system identification

In this study, a novel technique for fast transient stability limit assessment is proposed. In the proposed method, first the whole power system is approximated with a single machine equivalent model independent of the type and location of the fault, and its parameters are identified using on-line measured data. Next, a graphical index based on transient energy approach is presented for the transient stability limit. The proposed method is tested on a real 247-machine power network to all devices in service, and the load flow matched with real recorded data. Simulation studies show very promising results. Furthermore, they show that the defined stability index can be evaluated immediately after the fault clearing and can be accurately estimated even if does not exist any prior knowledge about the operating condition. This new method can be used for preventive security assessment in smart power systems. The results show that the proposed graphical stability limit can be used accurately as a measure for ‘distance to critical clearing time’ of relays.

Modelling demand response in organized wholesale energy markets

We propose a bi-level optimization model for demand response in organized wholesale energy markets. In this model, the lower level performs the economic dispatch of energy and generates the price and the upper level minimizes the total amount of demand response subject to a net benefit requirement. In an economic sense, demand response is a trade of ‘consuming rights’ instead of a sale of energy. Therefore it must be traded separately from the energy market. Although a bi-level optimization model is very hard to solve in general, we demonstrate that realistic power networks have characteristics that can be exploited to reduce the effective size of the problem instance. In particular, we transform the nonconvex net benefit test constraint to an equivalent linear form, and reformulate the nonconvex complementarity conditions of doubly bounded variables using SOS2 constraints. For realistic instances of the MPEC, we employ a three-phase approach that exploits the fast local solution from a nonlinear programming solver as well as LP-based bound strengthening within a mixed integer/SOS2 formulation. The model is tested against various data cases and settings, and generates useful insight for demand response dispatch operations in practice.

Wind Turbine Transformers Protection Method Against High-Frequency Transients

A new wind turbine transformers protection method against high du/dt switching transients taking place during a wind turbine operation is described. High-frequency transients, characterized by a high rate of voltage rise (du/dt) and overvoltage at the wind turbine transformer high-voltage terminals may be a result of operations of the turbine switchgear typically comprising a vacuum circuit breaker. The protection concept described in this paper is a follow up of research activities on a transients suppressing method dedicated for distribution transformers. The principle of operation of the protective device proposed and parameters optimization procedures for windmill transformers are provided in this paper. The performance of the device was verified by means of Alternative Transients Program-Electromagnetic Transients Program simulations. The physical device was built and experimentally verified by full-scale functional tests reflecting a real wind turbine power network.

Characterisation of large disturbance rotor angle and voltage stability in interconnected power networks with distributed wind generation

Wind generation in highly interconnected power networks creates local and centralised stability issues based on their proximity to conventional synchronous generators and load centres. This study examines the large disturbance stability issues (i.e. rotor angle and voltage stability) in power networks with geographically distributed wind resources in the context of a number of dispatch scenarios based on profiles of historical wind generation for a real power network. Stability issues have been analysed using novel stability indices developed from dynamic characteristics of wind generation. The results of this study show that localised stability issues worsen when significant penetration of both conventional and wind generation is present because of their non-complementary characteristics. In contrast, network stability improves when either high penetration of wind and synchronous generation is present in the network. Therefore network regions can be clustered into two distinct stability groups (i.e. superior stability and inferior stability regions). Network stability improves when a voltage control strategy is implemented at wind farms, however both stability clusters remain unchanged irrespective of change in the control strategy. Moreover, this study has shown that the enhanced fault ride-through strategy for wind farms can improve both voltage and rotor angle stability locally, but only a marginal improvement is evident in neighbouring regions.

Optimal allocation of capacitor banks in radial distribution systems for minimization of real power loss and maximization of network savings using bio-inspired optimization algorithms

Abstract In this paper, two new algorithms are implemented to solve optimal placement of capacitors in radial distribution systems in two ways that is, optimal placement of fixed size of capacitor banks (Variable Locations Fixed Capacitor banks-VLFQ) and optimal sizing and placement of capacitors (Variable Locations Variable sizing of Capacitors-VLVQ) for real power loss minimization and network savings maximization. The two bio-inspired algorithms Bat Algorithm (BA) and Cuckoo Search (CS): search for all possible locations in the system along with the different sizes of capacitors, in which the optimal sizes of capacitor are chosen to be standard sizes that are available in the market. To check the feasibility, the proposed algorithms are applied on standard 34 and 85 bus radial distribution systems. And the results are compared with results of other methods like Particle Swarm Optimization (PSO), Harmonic Search (HS), Genetic Algorithm (GA), Artificial Bee Colony (ABC), Teaching Learning Based Optimization (TLBO) and Plant Growth Simulation Algorithm (PGSA), as available in the literature. The proposed approaches are capable of producing high-quality solutions with good performance of convergence. The entire simulation has been developed in MATLAB R2010a software.

Plug-and-Play Distributed Algorithms for Optimized Power Generation in a Microgrid

This paper introduces distributed algorithms that share the power generation task in an optimized fashion among the several Distributed Energy Resources (DERs) within a microgrid. We borrow certain concepts from communication network theory, namely Additive-Increase-Multiplicative-Decrease (AIMD) algorithms, which are known to be convenient in terms of communication requirements and network efficiency. We adapt the synchronized version of AIMD to minimize a cost utility function of interest in the framework of smart grids. We then implement the AIMD utility optimisation strategies in a realistic power network simulation in Matlab-OpenDSS environment, and we show that the performance is very close to the full-communication centralized case.

도시철도 직류 전력량 계측을 위한 직류용 스마트미터링 시스템 개발 및 성능시험

DC urban railway power system consists of DC power network and AC power network. The DC power network supplies electric power to railway vehicles and the AC power network supplies electric power to station electric equipment. Recently, because of power consumption reduction and peak load shaving, intelligent measurement of regenerative energy and renewable energy adapted on DC urban railway is required. For this reason, DC smart metering system for DC power network shall be developed. Therefore, in this paper, DC voltage sensor, current sensor, and DC smart meter were developed and evaluated by performance test. DC voltage sensor was developed for measuring standard voltage range of DC urban railway, and DC current sensor was developed as hall effect split core type in order to install in existing system. DC smart meter possesses function of general intelligent electric power meter, such as measuring electricity and wireless communication etc. And, DC voltage sensor showed average 0.17% of measuring error for 2,000V/50mA, and current sensor showed average 0.21% of measuring error for ±2,000V/±4V in performance test. Also DC smart meter showed maximum 0.92% of measuring error for output of voltage sensor and current sensor. In similar environment for real DC power network, measuring error rate was under 0.5%. In conclusion, accuracy of DC smart metering system was confirmed by performance test, and more detailed performance will be verified by further real operation DC urban railway line test.

Modeling extreme values resulting from compromising electromagnetic emanations generated by an information system

Abstract Electromagnetic intelligence and attacks pose unacceptable risks for the security and safety of critical networks and more specifically the power network. In this paper, it is pointed out how the use of the excess model allows one to extrapolate the very high level of spurious compromising emanations induced by an information system in realistic power network models. It is shown that the design of appropriate protections and risk management methodologies can be enhanced thanks to the extreme value statistics .

Generalized $\pi$ Fortescue Equivalent Admittance Matrix Approach to Power Flow Solution

This paper develops a generalized admittance matrix approach in Fortescue coordinate system to solve unbalanced/unsymmetrical distribution networks including different number of phases. This generalized Fortescue π equivalent is defined in this paper for solving the heterogeneous phase, and thus Fortescue, network model. The performance of the approach is demonstrated in different model networks with number of nodes ranging between 168 and 14200. It is found that the current iteration method exploiting the decoupling in admittance matrix in Fortescue coordinate is substantially faster than the typical unbalanced three-phase solution in phase domain. The method has a significant potential for application in real time active power network management.

Reliability assessment of incentive- and priced-based demand response programs in restructured power systems

Fostering demand response (DR) through incentive-based and priced-based programs has always great impact on improvement of efficiency and reliability of the power systems. The use of DR lowers undesirable effects of failures that usually impose financial costs and inconveniences to the customers. Hence, quantifying the impact of demand response programs (DRPs) on reliability improvement of the restructured power systems is an important challenge for the independent system operators and the regional transmission organizations.In this paper, the DR model which treats consistently the main characteristics of the demand curve is developed for modeling. In proposed model, some penalties for customers in case of no responding to load reduction and incentives for customers who respond to reducing their loads are considered.In order to make analytical evaluation of the reliability, a mixed integer DC optimal power flow is proposed by which load curtailments and generation re-dispatches for each contingency state are determined. Both transmission and generation failures are considered in contingency enumeration. The proposed technique is modeled in the GAMS software and solved using CPLEX as a powerful mixed integer linear programming (MILP) solver. Both supply-side reliability for generation companies and demand-side reliability for customers are calculated using this technique.In order to simulate customers’ behavior to different DRPs in a real power network, the proposed DR model is used and evaluated over Iranian power network. In order to investigate the reliability effects of DRPs based on proposed reliability method, DRPs based on the DR model are implemented over the IEEE RTS 24-bus test system, and reliability indices for generation companies, transmission network and customers are calculated. Using proposed performance index, the priority of the DRPs are determined from view point of customers, generation companies, transmission network and the whole system in IEEE RTS.

Coordinated under frequency load and capacitor shedding for bulk power systems

Under frequency load shedding (UFLS) scheme has been widely implemented as a safety net to prevent system collapse following major disturbances which results in large mismatch between load and generation. However, voltage related issues post activation of UFLS stages are also vital concerns in preserving system stability and should be properly analysed. Fixed and switched shunt capacitors that are in service during normal operation for maintaining system voltage and dynamic MVAR reserve can generate surplus reactive power post operation of UFLS relays which may result in over voltage issues, generator under excitation and some undesirable conditions such as transformer saturation, ferro-resonance etc. This study addresses the need for coordinated under frequency load and capacitor shedding and its implementation approach to effectively preserve system stability following small and large disturbances. To confirm the feasibility of the approach, the proposed method has been used to design coordinated UFLS and under frequency capacitor shedding (UFCS) schemes for a real and actual power network. In addition, the proposed coordinated UFLS and UFCS scheme has been combined with automatic switching of shunt reactors to optimise the performance of the scheme.

A novel adaptive current protection scheme for distribution systems with distributed generation

An adaptive current protection scheme is proposed for the protection of power systems with penetration of distributed generation (DG). In this scheme, steady state fault current of the related power transmission lines is derived from steady state network equivalent reduction. Then, settings criteria for adaptive primary and backup protection are established on the basis of the steady state fault current. The simulated results of a realistic power network verify the ranges of adaptive primary and backup protection are immune to implementation of DG and the fault types. Furthermore, compared with traditional current protection schemes, the proposed method has extended the primary and backup protection regions considerably.

Spectral analysis of a real power network

We present a spectral analysis of a significant part of the Italian bulk power system. Spectral analysis applied to graphs is not a new tool, but analyses of the real world, especially of technological networks, currently are lacking. We show the spectral pattern of a high voltage power network, calculate the ratio between the smallest and largest non-zero Laplacian eigenvalue to check its synchronisability and calculate the epidemics threshold of a real HV power grid. We indicate the trade-off between sinchronisability and epidemic threshold as a major network design problem. Moreover, following Restrepo, Ott, Hunt, we propose the usage of the maximum eigenvalue of the adjacency matrix to assess a metric for the inter-dependency in the technological networks.

A New Practical Approach to Transient Stability-Constrained Optimal Power Flow

This paper presents a new, significantly improved, approach to formulate a global transient stability-constrained optimal power flow (TSC-OPF), where the sets of dynamic and transient stability constraints to be considered in the optimization process are reduced to one single stability constraint. This constraint is derived from dynamic information provided by the SIngle Machine Equivalent (SIME) method and is only expressed in terms of steady-state variables, which allows us to diminish the length of the time-domain simulation to be included into the global TSC-OPF to a single (initial) time step. In this way, the size of the resulting optimization problem is reduced to one very similar to that of a conventional OPF, overcoming the main drawback of global TSC-OPF techniques (its huge dimension) while maintaining its accuracy and improving its practical application to real power networks. Effectiveness of the proposal is demonstrated by numerical examples on the WSCC three-machine, nine-bus system and the Mexican 46-machine, 190-bus system.

Complex dynamics of Cournot game with bounded rationality in an oligopolistic electricity market

In order to accurately simulate the dynamic decision-making behaviors of market participants, a novel dynamic Cournot game model with bounded rationality of electricity market that considers the constraints of realistic power networks is proposed in this paper. This model is represented by discrete difference equations embedded within the optimization problem of consumption benefits. The Nash equilibrium of electricity market and its stability are quantitatively analyzed. It is found that there are different Nash equilibriums with different market parameters corresponding to different operating conditions of power network, i.e. congestion and non-congestion, and even in some cases there is no Nash equilibrium at all. Numerical simulations with the 2-node and IEEE 30-node systems are carried out to evaluate the dynamic behaviors of electricity market, especially the periodic and chaotic behaviors when the market parameters are beyond the stability region of Nash equilibrium.

QoS analysis of video streaming service in live cellular networks

The increasing computing and communication power of new terminals in mobile networks has converted these smart phones into Internet hosts. They can be used for traditional services, such as browsing or e-mailing, but also as platforms for specific new IP multimedia services like Push to Talk, streaming or IPTV. However, developing new services or porting existing popular ones should be done considering the target network in which the services will be deployed. This paper presents an analysis of video streaming, a service with real time requirements, on live mobile networks to validate current deployments. In this analysis we propose using smart phones as measurement devices in a way that enables them to capture customers’ experiences. An outstanding contribution of this paper is that real measurements are carried out in real networks, with real transmission power, real antennas and real network operators and protocol stacks. These measurements help to complete performance results obtained in simulations and emulations carried out in laboratories.

Power Hardware-in-the-Loop Testing of a YBCO Coated Conductor Fault Current Limiting Module

In recent years, good progress has been made in improving the quality and quantity afforded by the manufacturing process for YBCO coated conductors. As a result, several programs have started to develop electrical power applications like motors, transformers, and fault current limiters (SCFCL) with these conductors. High voltage resistive type SCFCLs may typically be assembled from modules connected in series and parallel to accommodate the required voltage and current levels. The limited length of such a SCFCL module simplifies the configuration, manufacturing, and maintenance. It also allows testing of these modules under laboratory conditions at reduced power levels. In order to test SCFCL modules under conditions they will experience in high voltage electrical networks, advanced test methods such as power hardware-in-the-loop (PHIL) can offer significant advantages. This method allows studying conditions such as voltage stability and severe system perturbations with the actual SCFCL module in the loop. Hence, the quench and recovery behavior of the SCFCL module can be investigated under conditions characteristic of real electrical power networks without elaborate experimental setups. This paper presents results from PHIL experiments with a SCFCL module consisting of an approx. 10 m coated conductor.

Dynamic modeling of the power market and analysis of its complex behavior based on a nonlinear complementarity function

Abstract In order to accurately simulate the dynamic decision-making behaviors of market participants, a new dynamic model of power markets that considers the constraints of realistic power networks is proposed in this paper. This model is represented by discrete difference equations embedded within the optimization problem of market clearing. Compared with existing dynamic models, the remarkable characteristic of the proposed model is twofold: it accurately reflects the process of market clearing by the Independent System Operator (ISO) while considering the inherent physical characteristics of power networks, i.e., the complex network constraints; and it describes the market condition that the generation and demand sides bid simultaneously. Using a nonlinear complementary function, the complex discrete difference dynamic model is transformed into a set of familiar discrete difference algebraic equations. Then, the complex dynamic behaviors of power markets are quantitatively analyzed. Corresponding to different operating conditions of power network, such as congestion or non-congestion, the Nash equilibrium of power markets and its stability are calculated, and the periodic and even chaotic dynamic behaviors are exhibited when the market parameters are beyond the stability region of the Nash equilibrium.