Showing papers by "Matti Lehtonen published in 2014"

A Distributed Algorithm for Managing Residential Demand Response in Smart Grids

Abstract: Demand response enabled by time-varying prices can propel the power industry toward a greater efficiency. However, a noncoordinated response of customers may lead to severe peak rebounds at periods with lower prices. In this regard, a coordinated demand response scheme can mitigate concerns about the peak rebounds. This paper presents a system-wide demand response management model to coordinate demand response provided by residential customers. The objective of the model is to flatten the total load profile that is subject to minimum individual cost of customers. The model is first formulated as a bi-level optimization problem. It is then casted into equivalent single-level problems, which are solved via an iterative distributed algorithm. Home load management (HLM) modules embedded in customers' smart meters are autonomous agents associated with the algorithm. In the algorithm, at first, HLM modules, in response to prices announced by the utility, optimize the daily operation of household appliances and send back the scheduled load profiles. Then, the total load profile is calculated and released by the utility. Thereafter, the HLM modules asynchronously update their schedule such that, given their least energy expenses, the most evenly distributed total load profile is achieved. The mutual interaction between the utility and HLM modules is continued to the point in which no further improvement is obtained. Convergence and optimality of the algorithm are proved.

Integration of Price-Based Demand Response in DisCos' Short-Term Decision Model

Abstract: Real-time electricity prices along with demand-side potentials can provide distribution companies (DisCos) with considerable financial and technical benefits compared to the conventional flat prices. This paper incorporates demand response in DisCos' short-term decision model in a real-time pricing (RTP) environment wherein consumers are charged based on hourly varying prices. Besides the hourly RTP sale prices, the established model deals with other DisCo's short-term activities including hourly purchases from the grid, commitment of distributed generation (DG) units, dispatch of shunt compensators, and invocation of load curtailments (LCs). The stochastic nature of wholesale market prices and customers load is also considered in the model. The model is a mixed integer linear programming (MILP) problem which can be easily solved via commercial software packages. The objective is to maximize the DisCo's expected profit while its revenue is limited by regulating bodies. A typical Finnish 20 kV urban distribution network is used to demonstrate the effectiveness of the established model. Simulation results are presented and discussed to investigate the impacts on both financial and technical aspects of using of RTP sale prices.

Distribution network reliability improvements in presence of demand response

Abstract: Demand response (DR) as a key integral part of the future smart grid is gaining a great and still growing focus of attention in nowadays electric power industries. However, many potential benefits of DR, although they have been envisioned to be significant, have not been yet thoroughly and quantitatively investigated. DR provides network operators with the opportunity to mitigate operational limit violations by load modification in place of load shedding when network reliability is jeopardised. This paper aims to comprehensively assess the potential impacts of DR on major attributes of service reliability in a residential distribution network. For doing so, firstly, load profiles for major residential appliances are extracted from grossly metered consumptions. Secondly, the flexibility associated with individual load profiles is estimated using the statistical data gathered through surveys and questionnaires. Thirdly, for every contingency, appliance level load profiles are modified based on their flexibilities such that the least possible interruption cost is realised. The obtained results are finally combined to calculate service reliability indices. The proposed framework is applied to Finnish distribution system and the obtained results demonstrate the efficiency and applicability of the proposed approach in real-world systems.

Circuit-Breaker Automated Failure Tracking Based on Coil Current Signature

Abstract: The online condition assessment of circuit breakers (CBs) has been increasingly requested by power utilities in recent years. Trip and close coil current (CC) signature is an effective and noninvasive parameter in CB online condition monitoring. This paper gives an insight into the impacts of the various failures on the coil current waveform, as well as on the CB operation time. The failures and their causes are categorized based on the outcome of these investigations. Finally, a new algorithm using trip/close CC is proposed to detect the mode and cause of CB incipient failures. In this study, the CC patterns are acquired by measurements carried out on (healthy and faulty) CBs of 72.5-kV and 24-kV rating voltages, having SF6 insulation medium, and are equipped with a spring-drive mechanism.

Demand response potential of residential HVAC loads considering users preferences

Abstract: Domestic Heating Ventilation and Air-Conditioning (HVAC) loads are among the most flexible residential loads for possible demand response (DR) applications. This paper aims at assessing the DR potential of HVAC loads in smart grids considering users' temperature preferences. For doing so, at first, a mathematical formulation is developed to evaluate the flexibility of HVAC loads. The model, by adjusting the HVAC load, intends to either maximize or minimize electricity consumption during specific hours of a day while the user thermal comfort is intact. The proposed model is then applied to a medium massive structure house in Helsinki, Finland. The study is performed for different seasons and the associated DR potential is evaluated. The simulation results showcase the significant potential of HVAC loads in both consumption reduction and load increment. It is shown that the DR potential can be significantly affected by temperature dead-bands representing consumers' willingness to accept change in their comfort. It is also demonstrated that installing even a small thermal storage capacity may lead to a greater value-added DR potential.

Impacts of Time-Varying Electricity Rates on Forward Contract Scheduling of DisCos

Abstract: Time-varying electricity rates enable demand-side potentials, which provide an opportunity for distribution companies (DisCos) to hedge against the financial risk imposed by volatile spot market prices and uncertain customers' load. In particular, time-varying rates can be effective alternatives for at least a portion of costly forward contracts. This paper establishes a stochastic framework to determine optimal forward market purchases under time-varying rates. Various electricity rating strategies with different time intervals covering flat, time-of-use, and real-time pricing schemes are considered. The objective of the framework is to maximize DisCo's expected profit while the exposure risk is restricted to a predetermined level. The risk is modeled using the conditional value at risk approach. The elastic behavior of demand is taken into account via the price elasticity matrix model. The proposed framework is formulated as a mixed-integer linear programming problem which can be easily solved through commercially available solvers. The effectiveness of the developed methodology is examined through comprehensive case studies based on real data from Finland. A detailed comparison on the scheduling of forward contracts under different rating strategies is also provided.

Customer Interruption Cost in Smart Grids

Abstract: Customers participating in demand response programs known as active customers provide significant flexibility in demand, thereby causing a meaningful reduction in customer interruption cost (CIC) during critical periods. This letter illustrates the impact on the amount of CIC of active customers' flexible demand realized by smart grid technologies. The focus of this letter is on residential electricity use. The study is applied to actual data from Kainuu, Finland.

Photovoltaic power generation hourly modelling

Abstract: With the increase of solar power production, the modelling of this generation has to be considered in distribution network planning and operation. The purpose of this paper is to present a model that can estimate the hourly energy produced by a solar module in different sunshine conditions. The output power of a solar module is mainly dependent on solar radiation, temperature of the solar cell, and the technical properties of the module. The most complex of these is the solar radiation and how it changes in real sky and weather conditions. To model the solar radiation in southern Finland area, measured hourly long term radiation and sunshine data was obtained from the Finnish Meteorological Institution. With these data a regression model for solar irradiation was constructed and used for the power generation model. The solar irradiation model manages to estimate the irradiation quite well especially for a horizontal surface.

Harmonic load of residential distribution network — Case study monitoring results

Abstract: The non-linear loads in domestic applications are becoming more common as the power electronic converters are being implemented more widely. This can lead to more significant load current distortions. This paper is presenting a case study results to describe the expected harmonic distortions and to provide models to estimate their magnitudes during day for different times of year. The results presented are from a residential area low-voltage network, which is also providing the heating power for the households. The discussion is presented about the daily harmonic current profiles, which are presented to vary to great extent, harmonic to harmonic and for the different times of the year. The daily profiles of the harmonics with more stable characteristics are described using daily patters trend models.

Dynamic Thermal Modeling of MV/LV Prefabricated Substations

Abstract: With the expansion and infilling of urban areas, the demand for electric power is driving the design and capacity of distribution substations to their thermal limits. Distribution transformer substations are increasingly required to be compact, reliable, safe, and intelligent. To efficiently utilize city space and to support the intermittent load flows imposed by smart-grid features, such as distributed generation, the transformers are expected to operate close to or occasionally over their ratings, with stalled or little air circulation inside the safety enclosure. Dynamic thermal models with physically validated convection and radiation heat-transfer components are essential for the real-time thermal rating of substations. Natural convection via the air inside the cabin to the outside ambient air plays the major role in cooling down a transformer. In this study a scale model of a prefabricated substation is examined to draft a numerical solution which is based on stack ventilation principles. A clear and expandable first principle approach is used to quantify heat transfer through ventilation openings. Measurements from actual cabins and 3-D finite element method simulations are used to validate the numerical model.

Simple fault path indication techniques for earth faults

Abstract: Fault indication is the key element in any fault management system. In case of short circuits, fault indication is straightforward, but especially in ungrounded and compensated neutral networks, the single phase fault currents are typically so small that simple fault current threshold detection hardly gives any satisfactory results. The problem can be solved by combining neutral voltage measurement to the sum current measurement using directional relay characteristics. However, the requirement of both current and voltage measurements makes the solution cost prohibitive especially when retrofitting in the existing secondary substations. Hence, there is the need for a simple but reliable earth fault indicator which is based on current measurements solely. This paper takes the above mentioned challenge and compares a number of possible fault indication solutions which are using local current measurements at the secondary substations only. The methods presented use either the sum current measurement directly, or are based on the comparison of the measured phase currents using symmetrical components.

A summary of the recent extreme weather events and their impacts on electricity

Abstract: North America and Europe have been benefiting of high level of electric power reliability thanks to their robust transmission and distribution systems. Nonetheless, due to the increasing number of extreme weather events, such as; hurricanes, floods and winter storms, there have been many large scale black outs during the last decade. As the societies get more and more dependent to the continuous electric power, the consequences of power interruptions get bigger. This paper summarizes the impacts of some major extreme weather events on the electricity and focuses on the Storm Gudrun of 2005 in Sweden. Protection practices of the electric power infrastructure against natural hazards are presented. Moreover, the changes in the Swedish electricity law regulating the customer compensation scheme due to experiences of Gudrun are briefed.

Criticality Analysis of Failure to Communicate in Automated Fault-Management Schemes

Abstract: The effectiveness of various functions in a smart distribution grid relies on a reliable communication system. These functions are normally developed based on specific automation schemes. The majority of automated fault-management schemes (AFMSs) requires a dependable communication system to accomplish their intended functions. In a smart distribution grid, the AFMSs are responsible for automatic fault detection, location, isolation, and service restoration activities. Failure to communicate among various communicating devices involved in the AFMS may have a considerable impact on the reliability of electricity service delivered to the customers. This paper aims to quantitatively evaluate the reliability impact of failure to communicate in the AFMS. A highly loaded urban distribution network equipped with AFMS is utilized for directing the quantitative reliability case studies. The study results show that the service reliability can be significantly affected if the communication system fails to operate successfully.

Fault location, isolation and restoration in a city distribution network

Abstract: Active network management is based on distribution network automation systems like SCADA/DMS, distribution automation, microgrids, smart meters and home energy management systems. In fault situations, a lot of data is available and received to the control centre. To improve the fault management and in order to get the best benefit from high volume of monitoring data, the adaptation of new distribution network automation concepts are required. Automation systems will be integrated already on field to merge information for holistic distribution automation. Existing systems will be extended by additional functionalities. These functionalities can be located in different levels of control hierarchy: in Substation automation, in Distribution Management System (DMS) or in Network Information System (NIS). Helen Electricity Network Ltd., Aalto University and Tekla Corporation have been working together to help the control centre in fault management process to improve the reliability of supply. The idea is to combine network data, topology and distribution automation data and develop new fault management features, functions for automatic fault location, isolation and recovery in city distribution network.

Cyclone Dagmar of 2011 and its impacts in Finland

Abstract: The increasing frequency of the natural disasters during the last decade and their severe impacts on the electric power arose debate and led questioning the electric power reliability of the European countries. The Cyclone Dagmar of 2011, or locally known as Tapani storm, was a shocking event for all Scandinavia in terms of its impacts on the electricity supply and the related services. Although Finland had been thought to be enjoying highly reliable electric power, after 2011 the authorities started to go through radical changes to improve the supply security of the country and the Finnish Ministry of Employment and the Economy introduced a 15-year plan to avoid such undesirable outcomes of the possible natural disasters that might occur in the future. This paper summarizes and presents the events that were caused by the storm, and the storm recovery efforts spent after the incident.

Dynamic thermal state forecasting of distribution network components: For enhanced active distribution network capacity

Abstract: This paper proposes and investigates a framework for day-ahead hour-by-hour thermal state forecasting of distribution network components. The method takes into consideration the uncertainty brought about by distributed generation and load demand forecasting. Potential risks are evaluated so that contingency measures can be planned with a clear knowledge of tolerance to temporary overloading scenarios.

Estimating the harmonic distortions in a distribution network supplying EV charging load using practical source data — Case example

Abstract: This paper presents a case example of simulations of the total harmonic load currents when the electric vehicle charging is added to the residential area distribution network workday harmonic current load profiles. For the distribution network harmonic current data, actual recorded harmonic daily load patterns of a residential area low-voltage network are used. Vehicle behavior and daily utilization estimates are based on a traffic statistic study results. The electric vehicles harmonic current loads were measured for 4 different commercially available vehicles. Based on the traffic survey, the total EV charging demand has been simulated with Monte Carlo methods used and total distribution network harmonic currents presented. Results of the simulation are focusing on the 3rd and 5th harmonic currents. It will be presented that it is highly likely that there will be significant harmonic cancellation of the harmonic currents also for the lower order of harmonics present. Results of the study indicate rather minor increase in the harmonic currents due to EV charging.

Cross-Correlation Method for Power Arcing Source Monitoring System

Abstract: Power arcs do not only cause important economic loss, but also lead to serious deterioration of the entire power system equipment. With the aging of the distribution networks, the development of power arcs detection and location techniques has been paid more attention. Once an arcing fault has been detected at a monitoring station its location is obtained from the electromagnetic radiation signals. The most common location techniques are based on Time Difference of Arrival (TDOA), Directional Finding (FD) and Propagation Attenuation (PA). In this paper, cross-correlation method in connection with TDOA is used to locate power arcing faults. In the experiment, strategically placed antennas and the arrival time’s delay of dominant component of the wide-band electromagnetic signals radiated from the sources are used. The power electric arc was produced by a tree leaning on a current conducting cable. The experiment proves that cross-correlation method combined with TDOA can be used to locate power arcs accurately.

Harmonic distortions of multiple power factor compensated EV chargers

Abstract: Electric vehicle chargers are mostly power converters with power factor correction and waveform shaping features included. Different vehicle makes and models will likely have different charger control and topology. While the charger charging currents have generally low AC waveform distortions, including variety of chargers at the same time can provide even further lower harmonic currents. This is due to harmonic cancellation brought by different harmonic patterns of different chargers. The present paper analyzes the sine distortions and the sum of the harmonic currents of multiple harmonic sources of such type. The inputs for the analysis are the measurements of the commercial EV's harmonic charging current patterns. It will be shown that remarkable harmonic cancellation will be likely if different vehicle types are included in the pool of vehicles charging at the same time. The results presented here for the EV chargers would be valid also for other similar mains interface converters.

Electric vehicle charger load current harmonics variations due to supply voltage level differences — Case examples

Abstract: Modern switching power converters responsible for AC to DC conversion use advanced control methods that guarantee close to unity power factor and sine-like current waveforms. The level of harmonic distortions of such converters that employ power factor correction (PFC) can be rather low and remain in ranges of few percent. Such converters are employed also as chargers as electric vehicles (EVs). When EVs are connected to the low-voltage grid for slow charging, the AC supply voltage levels of the different charging locations are likely to vary. This can also provide effect to the load current waveform distortions, affecting the magnitude and phase shift angles of the individual harmonics. In this paper a discussion is presented on the effects of the AC supply voltage variations to the load current harmonic patterns and summation of harmonic currents when the chargers are used at different AC voltage levels.

Load shifting in the existing distribution network and perspectives for EV charging-case study

Abstract: One of the traditional load shifting motivations in use for long time already is the two-level time-of-use tariff scheme. The nighttime lower energy has been known to provide lower cost for the homeowners who are using electric energy for space heating. The measurements of residential areas, presented in this paper, confirm that the load shift towards the times of lower pricing is used by many homeowners to benefit from this. This brings high load power peaks for the distribution network for some shorter periods after the pricing is changed. When introducing EV charging to the household loads, homeowners are likely to shift the start of EV charging time also for providing the lowest-price charging, as the cost if energy used for the EV can also be remarkable. The additional EV charging peak will be investigated in this paper and its effect to the existing network load power peak observed. Load data from existing residential network monitoring will be used as a basis for the comparison, where the time-of-use tariffs already have great impact. It will be shown that if the fixed-time tariff change is applied to domestic EV charging, this will result in rise of the short-time peak power demand peak in the network. For the distribution network operation, the uncontrolled EV charging could provide more relief than the simple two-tariff pricing case.

Performance Evaluation of PD Monitoring Technique Integrated into Medium Voltage Cable Network for Smart Condition Assessment

Abstract: Online condition assessment of medium voltage (MV) cables is a valuable tool to reduce the probability of forced outages in the distribution network. Partial discharge (PD) diagnostics is being used as an efficient methodology to predict the deteriorating insulation of the power components. In this paper a comprehensive condition assessment solution is proposed for cable networks. Low cost non-intrusive devices are installed at each critical location in such a way as to monitor the entire feeder. The Direction of arrival (DOA) technique is used to monitor the PD activity. The idea of a data-base is developed to process the PD data captured by sensors distributed along the cable feeder. The performance of integrated sensors is evaluated using ATP-EMTP simulation. The proposed system can be used for both continuous and periodic monitoring. However in order to overcome the limitations of the conventional periodic and continuous monitoring, a hybrid monitoring system is proposed to be applied for the real networks.