Showing papers by "Matti Lehtonen published in 2013"

A Stochastic Framework for Short-Term Operation of a Distribution Company

Abstract: This paper presents a stochastic framework for short-term operation of a distribution company (disco). The proposed framework optimizes disco's operational decisions in two hierarchical stages. The first stage, called day-ahead operation stage (DAOS), deals with the operational decisions on purchases from the day-ahead market and commitment of distributed generation (DG) units. The objective of this stage is to minimize the expected operating cost while the financial risk exposed by uncertain real-time prices and loads is restricted to a given level. The model associated with this stage is based on the mixed-integer programming (MIP) format. The second stage, named real-time operation stage (RTOS), deals with disco's activities in real-time. In RTOS, decisions are made on real-time market transactions, dispatch of online DGs, and invocation of load curtailments (LCs) such that the expected operating cost is minimized. This stage is formulated as a nonlinear programming (NLP) problem. To investigate the effectiveness of the developed framework, it is applied to a typical Finnish 20-kV urban distribution network.

Clustering of Connection Points and Load Modeling in Distribution Systems

Abstract: The lifetime of transmission and distribution power systems is long and thus, long-term plans are needed for their successful development. In generating long-term scenarios, the starting point is the analysis of the present electricity consumption. The data of electricity consumption will become more exact by the end of 2013, when hourly based automated meter reading (AMR) consumption data will be received from each customer in Finland. The amount of data is huge and powerful analysis methods are needed. This paper presents a method for clustering the electricity consumptions using principal component analysis (PCA) and K-means clustering. AMR data of 18 098 customers from two city districts of Helsinki, Finland is applied for a case study reported in this paper. A multiple regression analysis is also carried out on the two largest clusters to find the most important explanatory factors for the load modeling. The interpretations of the clusters and the plausibility of the regression coefficients are considered very important. Five distinct and meaningful clusters are found. The regression models give interesting insights into the explanatory factors behind electricity consumption. The models of the main customer groups assist the distribution system operator (DSO) in the long-term development of the power system.

Parameters Identification and Modeling of High-Frequency Current Transducer for Partial Discharge Measurements

Abstract: Rogowski coil (RC) is a low-cost, air-cored, and flexible induction sensor for nonintrusive condition monitoring and thus can be used in a variety of applications. In this paper, a lumped parameter model of RC is presented and an experiment-based methodology is developed to determine its parameters. The performance of the RC is analyzed for detection and measurement of high-frequency (pulsed) signals such as partial discharge (PD) current pulses. A simple and efficient technique of numerical integration is adopted to avoid the conventional type of expensive and complex design analogue integrators. RC is modeled and simulated in the alternative transient program-electromagnetic transient program environment. The designed coil is tested to measure PDs in the laboratory. Simulated and experimental performance of RC is compared with a high-frequency current transformer. This comparison shows a good match and, hence, validates the design of RC for PD applications.

A review of the harmonic and unbalance effects in electrical distribution networks due to EV charging

Abstract: For wide use of electric vehicles (EVs), there are different aspects of the electric power system to consider for making it ready for the increased load by battery charging. The topics include power production, peak load management, distribution transmission capacity but also distribution network power quality and many more. This paper presents an overview on the likely power quality impacts in the distribution networks associated with EV charging. Based on a literature review, focus is especially put on harmonics and load unbalance in the network. Most relevant papers observing these topics are presented summarizing their contribution. The power quality aspects in distribution networks discussed here are not often presented within analysis of permissible EV penetration levels. Harmonics or voltage unbalance and effects associated with these could introduce additional limits to the EV charging capacity for the distribution networks. Therefore the analysis on the EV charging influence on these power quality topics requires also high-priority discussions before drawing conclusions on the distribution networks capabilities.

The Smart Solution for the Prediction of Slowly Developing Electrical Faults in MV Switchgear Using Partial Discharge Measurements

Abstract: An electrical fault in switchgear results in interruption of power supply, damage to equipment, and poses a hazard to personnel. This paper focuses on the detection of slowly developing faults leading to internal arc, using online monitoring technologies in medium-voltage switchgear. Unconventional radio-frequency (RF) techniques for discharge measurement are highly attractive but have not been widely applied in the industry due to their ineligibility to quantify actual discharge. On the basis of various benefits, a new application of a differential electric field ( D-dot) sensor for partial-discharge (PD) measurements has been introduced in this paper. The reliability of the sensor has been confirmed through comparison with a commercial high- frequency current transformer. An attempt has been made to quantify the apparent charge of online PD measurements. The energy of signal captured by the D-dot sensor has been compared with the apparent charge quantity calculated from current pulse measured by the conventional method. A second degree polynomial relation exists between the cumulative energy and apparent charge. It has been shown that when apparent charge is plotted against the cumulative energy of the RF signal for a number of pulses, defects can be separated on the basis of cluster positions within the scatter plot.

Priority Assessment of Online Monitoring Investment for Power System Circuit Breakers—Part I: Qualitative-Quantitative Approach

Abstract: Recognition of the circuit-breaker's (CB) vital role in reliable operation and protection of power systems (i.e., during energization, disconnecting loads, and clearing faults), helps to understand the requirement and importance of its maintenance management as well as maintenance scheduling. Condition-based maintenance (CBM), employing online monitoring (OLM) of CBs, has been long reported as the most practical maintenance policy on the power system CBs. There are a large number of CBs in a power system to be monitored; however, to address the financial limitations for OLM implementation (in utilities), a method will be proposed to find the most critical breakers for enforcing effectively the CBs CBM planning in this power system. This paper employs some qualitative and quantitative criteria to develop this method. While, the former criterion is deals with through assessing the condition of CBs through fuzzy sets theory. And the latter criterion is dealt with through the evaluation of CBs' influence on the overall system reliability (through introduced indices). This method is applied on a sample transmission substation with a breaker-and-a-half configuration. Numerical analysis of this simulation results demonstrates how this method can be employed to prioritize the CBs for OLM.

Pre-emptive arc fault detection techniques in switchgear and controlgear — Part II

Abstract: The significant benefits of preemptive arc-flash protection and the online condition monitoring of electrical equipment are quite well known. Our continuing research focuses on the development of new advanced sensor technologies that are cost-effective, reliable, and efficient for the early detection of faults in order to predict impending arc-flash occurrences in mediumvoltage and low-voltage switchgear and controlgear. More extensive and detailed measurements regarding significant defects that lead to an arc-flash event have been completed since the original work in Part I was completed. A more detailed analysis of the results of this additional testing is presented in this paper. It has been documented that the two major noncontact causes that lead to an arc-flash event in switchgear are insulation degradation and thermal stresses. This paper covers the detailed measurement results under both of these conditions. New sensor technologies for both the partial discharge measurement and the thermal detection are introduced and evaluated. An effective signal processing technique, which is needed for extracting the essential indication of a developing fault, is also presented. Finally, this paper outlines how a preemptive arc detection system can be connected to protection, the programmable logic controller, or the supervisory control and data acquisition.

Current harmonics of EV chargers and effects of diversity to charging load current distortions in distribution networks

Abstract: Charging of electric vehicles (EVs) is expected to bring a healthy addition of load for the distribution networks. The residential networks where the EV owners would charge their vehicles after returning from daily activities would especially be subjected to high load increase. As EV charger is a powerful nonlinear load rather large harmonic currents can be present during the EV charging. This means a significant increase also to the current harmonics. Analysis of the quantities of the harmonic currents is necessary for guaranteeing the distribution network operation that would meet the power supply standards. In this paper, the EV charging measurement results are presented and analyzed with focus on the current waveform distortions. Different EVs are analyzed for the current harmonics present during the slow rate home charging. For the modeling of the EV charging loads in the networks, discussion is presented on the harmonic currents summing and cancellation effects. The results presented in the paper can be further used for modeling of the actual harmonic loads of the EVs in the distribution networks.

Effect of Terminating Resistance on High Frequency Behaviour of Rogowski Coil for Transient Measurements

Abstract: Partial Discharge (PD) current measurements have been widely used to assess the insulation condition of power components. Rogowski coil is a low cost current sensor specifically meant for high frequency transient diagnostics such as PDs in high voltage (HV) power apparatus. This paper presents the design of Rogowski coil for transient measurements. For the designed prototype of Rogowski coil, correct selection of terminating resistance is of high importance to calibrate its response for accurate measurement of the PD current waveforms. Effect of terminating resistance for the coil model is analysed based on theoretical, practical and simulated investigations. Simulated model of the coil is used to determine the behaviour of induced current through non-physical inductive and capacitive elements of Rogowski coil. The Electromagnetic Transient Program-Alternative Transient Program (EMTP-ATP) is used to implement and to analyze the model of Rogowski coil. The comparison of practical and simulated results validates the designed model of the coil. The paper explores EMTP-ATP as an efficient tool to analyze and modify the transient behaviour of Rogowski coil for high frequency pulse measurements.DOI: http://dx.doi.org/10.5755/j01.eee.19.7.2070

Priority Assessment of Online Monitoring Investment for Power System Circuit Breakers—Part II: Determination of Optimum Number

Abstract: The proper and reliable operation of circuit breakers (CBs) directly affects the reliability and continuity of electricity services in a power systems. Condition-based maintenance (CBM) is the best economic CB maintenance strategy through its online monitoring (OLM) possibilities. Since there is a large number of CBs in a power system, to equip all CBs with OLM systems is not economically justifiable however. This paper, as the second part of a two-part paper, presents an effective optimization framework to find the optimal number of CBs to be monitored online as the kernel of CBM process. To prove its feasibility, the proposed approach is applied to the IEEE Roy Billinton Test System, which includes 32 CBs. The impacts of interest rate and load interruption cost variations as well as nonoptimum selection of CBs for monitoring on the results are also discussed in this paper. The results of numerical analyses of the case study presented in this paper demonstrate how the proposed method can be employed for maintenance management of power system CBs.

Electricity Distribution Network Planning Algorithm Based on Efficient Initial and Radial-to-Full Network Conversion

Abstract: Electricity distribution networks, the means of transporting electricity between the high voltage transmission networks, end users, local generation and storage, are an important part of today’s infrastructure. Rapid upgrading and development are required to cope with their more active role in enabling the integration of distributed generation and demand side management. To aid the laborious task of planning real distribution networks, an extensive network planning algorithm has been developed for use in a Network Information System (NIS). This paper focuses on two important aspects of the full algorithm: an efficient method to generate an initial network that is close to optimum, and the conversion of an underlying radial network into a fully or optimally backed-up and switched network. Backup refers to reserve connections that provide an alternative feed during contingencies. A reproducible initial network example and further examples for comparison are given for medium voltage (MV) distribution network topologies between primary (HV/MV) substations and secondary (MV/LV) substations. The secondary substations may supply low voltage (LV) network, industrial loads or even form the connection point to self-contained microgrids. The paper provides methodology that is specifically developed for planning distribution networks with a fully embedded treatment of reliability

Online partial discharge diagnostics in medium voltage branched cable networks

Abstract: Partial discharge (PD) detection and localization in medium voltage (MV) cables connected in branched network configurations, is difficult by using the conventional methods. It is due to interconnected cables of different lengths and impedance which make the interpretation of PD signals quite complex as compared to single cable routes. This paper presents an online technique of PD localization for branched cable network. PD activity emits current pulses propagating away from the discharge location. Polarity of the detected PD pulses with reference to polarity of the supply voltage can determine the direction of arrival of PD pulses. Comparison of the polarity of PD pulses captured at different cable branches identifies the faulty section of the cable network. Evaluation of direction of arrival is based on directionally calibrated installed sensor. In this paper induction sensors are employed for implementation of the proposed technique. Integration of the proposed technique over a wider network can be very useful as an efficient diagnostic tool for improving the reliability of power supply.

Evaluation of power outage costs for industrial sectors in Finland

Abstract: Estimating the customer interruption costs is a challenge for the electric power society. In this paper the results of a customer survey on power outage costs for industry sectors conducted in Finland have been presented. In addition, the problem of strategic responses has been investigated and an elimination method for the zero and extreme responses has been proposed. (4 pages)

An improved technique to determine the wave propagation velocity of medium voltage cables for PD diagnostics

Abstract: Investigation of wave propagation characteristics for high frequency pulses is an important aspect for the partial discharge (PD) diagnostics in medium voltage cables. Precision in determining the electromagnetic wave propagation velocity in the cables determines the accuracy of developing cable model and localization of partial discharge initiation site. It has been observed that commonly used time domain reflectometry (TDR) and time difference of arrival (TDA) methods pose certain limitations while finding out the propagation velocity of the cables. This can cause serious inaccuracy during localization of PD faults. This paper presents an alternative quarter wave length (QWL) method in frequency domain to measure the propagation velocity. It is based on transmission line model of the cables and eliminates the practical limitations of TDR and TDA techniques. The comparative assessment of these methods is presented based on experimental measurements. The comparison asserts the improved performance of proposed technique. This improved technique can be used to enhance the diagnostics capability for power networks.

Effect of Climate Change on Transformers Loading Conditions in the Future Smart Grid Environment

Abstract: The steady-state calculations are performed using IEC guidelines to determine the hot spot temperatures of distribution and power transformers in the worst projected Finnish environment due to long summer periods. Moreover, the effect of increase in winding resistance due to increase in ambient temperatures has been taken into account. The primary objective of the research is to investigate the possible extreme circumstances due to climate change. It is concluded that the power and distribution transformers should be progressively de-rated under such circumstances for their safe operations, which will not only prove cost-effective for utilities but also improve the reliability of the power supply to their valued customers in the challenging future smart grid environment.

Customer Interruption Costs Estimations for Service Sectors via Customer Survey Method: A Case Study

Abstract: The estimations of the economic impacts of the unwanted power interruptions have been a popular area of interest among the electric power society for the last three decades. In this paper the authors put forward estimations of customer interruption costs for service sectors by the aid of a customer survey conducted in Finland. The customer survey method has been criticised and a necessity of a more sophisticated future work have been emphasised

Signal processing of PD measurements to predict arcing faults in MV switchgears

Abstract: Arcing faults in MV switchgear cause serious hazard to personnel, significant damage to equipment, and often serious process interruptions. Many of the faults develop slowly, e.g. because of insulation degradation or loose connection. An interesting research question is whether these developing faults could be detected before they escalate into devastating high-power faults. Detection of partial discharges (PD) or monitoring of temperature has been suggested in on-line monitoring systems. In this research, a switchgear panel has been subjected to PD in the laboratory and measurements have been captured by different sensors and recorded by high frequency oscilloscope. Generally, the on-line signals are suppressed by high frequency noise, therefore, the de-noising of PD measurement is of paramount importance to get reliable arcing fault prediction results. The discrete wavelet transform (DWT) to de-noise such PD signals has been employed in this paper. Time domain and frequency domain comparisons of original and de-noised PD signal reveal the significance of this technique for arcing fault prediction in medium voltage (MV) switchgears.

Distribution Network Topology Planning Using Life Cycle Cost driven Cost Surfaces in the Internodal Parameter Computation

Abstract: This paper presents a method for using the most accurate available geographic data in network topology planning. The process is an iterative process, where an initial network topology is first generated in order to acquire certain topology related parameters for each node-to-node connection. Then, node-to-node optimal routes are considered through individual cost surfaces, generated with varying topological and static geographic inputs. A near-optimal network topology is then constructed, now also considering the geographical cost drivers and constraints. In the topology the back-up connections and network sectionalization devices are also considered from the point of view of cost-optimality. A study from a real network proves the potential of the developed methodology to be able to cope with realistically sized networks

A new approach for long-term electricity load forecasting

Abstract: Long-term electricity load and price forecasts have become critical inputs to energy service provider (ESP) decision makings in restructured environments. This paper presents a three-stage hierarchical approach for long-term electricity load forecasting. These stages are called yearly trend model (YTM), weekly trend model (WTM), and daily trend model (DTM). The first stage fits an appropriate function to data and extracts its yearly trend. The weekly and daily trends are then extracted using the Box-Jenkins method in WTM and DTM, respectively. For doing so, candidate trends are identified using auto correlation function (ACF) and partial auto correlation function (PACF) plots. Then, Akaike information criterion (AIC) and Schwarz information criterion (SIC) are used to select the best-fitted trends. The different behavior of weekends and night times is captured using dummy variables. The obtained yearly, weekly, and daily trends are finally used for electricity load forecasting.

Modeling electric vehicle charging flexibility for the maintaining of power balance

Abstract: Maintaining the power balance between electricity consumption and generation might become more challenging in the near future due to intermittent electricity generation. The decrease in flexibility on the generation side may at least to some extent be compensated by managing demand. Therefore, this paper concentrates on modeling the flexibility electric vehicles (EVs) are able to offer to the maintaining of power balance over a typical weekday. The modeling rests on real mobility behavior data utilized in the dynamic simulations of an EV fleet. Of particular interest are the hourly powers and energies available when a portion of the EV battery capacity is reserved for the maintaining of the balance. Employing the estimated flexibility, EV contribution to the power system frequency control and hourly regulation is analyzed in the Finnish power system. The results indicate that EVs are able to contribute to the maintaining of power balance, but this depends greatly on the reserved energy content of the batteries.

An Earth Fault Location Scheme for Isolated and Compensated Neutral Distribution Systems

Abstract: This paper proposes a scheme to locate an earth fault in an unearthed or a compensated neutral medium voltage (MV) network using the transient signal recorded from MV/LV substations. The algorithm applies continuous wavelet Transform (CWT) to locate the dominant charge transient frequency and then fast Fourier transform (FFT) to extract coefficients to be used in the fault location scheme. The fault location scheme managed to identify the correct path of fault location towards the position of the fault using the transient signal recorded from secondary side of the LV substation transformers. The results from intensive simulations and experiments in actual distribution network are also presented in this paper. The final results show that the fault location scheme is able to locate and identify the correct fault location successfully.

Optimization of distribution automation in urban networks

Abstract: Distribution automation (DA) is one of the key elements in mitigating the inconvenience caused by network faults in city distribution networks. DA is targeted at shortening the interruption time once the fault has occurred. In this paper, the planning approach relies on the comparison between the customer interruption costs (CIC) and coverage of DA in urban networks. Also the effects of substation outages to the benefit of distribution automation are evaluated. The major outcome of the study is the creation of flexible and accurate optimization of distribution automation in urban networks. The method is especially suitable and usable for those utilities, which has both network and customer data in the Geographical Information System (GIS) or Network Information System (NIS).

Markov model based assessment for redundancy mitigation in high voltage grids using demand response

Abstract: Demand response (DR) has become a key feature of the future Smart Grid. This paper assesses the potential of DR in mitigating the redundancy requirement of a high voltage (sub-transmission) grid. DR is considered as a redundancy alternative, activated by network contingencies. The end use appliance consumption pattern along with the load disaggregation technique is applied to determine the achievable DR. The comparison of outage cost for future load is adopted as an assessment method; this comparison is between non-investing in the network, and use of DR as a redundancy alternative. In the presence of DR, novel reliability models are developed in order to find the outage cost. The analysis conducted on a typical Finnish sub-transmission network indicates that the redundant capacity of the network proportional to DR capability can be mitigated. The employment of released redundant capacity for the increasing load will escalate network efficiency. As a consequence investments will be avoided

A new approach for AC state estimation based on a linear network model

Abstract: This paper presents a new and efficient formulation for the state estimation (SE) which can be solved in one single shot of computation. The proposed approach adopts line flows and the square of voltage magnitudes as the problem state variable and considers both active and reactive power quantities. The objective function is to minimize the weighted sum of the least square values of measurement residuals. The nonlinearity associated with line losses is left by assuming these values as slack variables. Numerical studies are conducted through two standard networks. In order to compare the performance of the proposed method, the conventional weighted least square (WLS) state estimation with Gauss-Newton AC power flow is also accommodated. Simulation results demonstrate that the proposed method outperforms the conventional one particularly from the speed while keeping the accuracy in a reasonable range.