Eero Saarijärvi

Impact of MV Connected Microgrids on MV Distribution Planning

The planning and development of distribution networks with a substantial penetration of microgrids connected to the medium voltage (MV) network form the main themes of this paper. The impact of microgrids is assessed in terms of their effect on optimal network topology, losses, reliability, reserve connections, network upgrade and expansion savings. The earning base of the distribution system operator also comes under scrutiny. A suburban MV cable network is planned using a network planning algorithm developed by the authors, first with optimal routing for demand-only nodes and then with a 33% penetration of randomly located microgrids. The network is then expanded to meet the requirements of a future planning horizon, in order to compare the expanded and upgraded optimum MV network topology with and without microgrids. Apart from visually depicting the topological differences, the savings such microgrids can give to the long term distribution network investment and running costs are quantified in terms of the investment costs, loss costs and interruption costs. When networks are planned with optimal rather than full backup, the introduction of microgrids is shown to have a considerable saving impact on all cost components except the cost per unit power transfer in the distribution network.

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

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.

Harmonic load of residential distribution network — Case study monitoring results

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.

Statistical analysis of large scale wind power generation using Monte Carlo Simulations

As more wind power generation is installed, the effect of wind power on the electric power system is becoming increasingly important. This paper presents two time series models that can be used in Monte Carlo simulations to assess the risk of very high or low wind speeds occurring contemporaneously in multiple locations. The suitability of the models is assessed for existing measured locations and new non-measured locations. The simulation results are verified against measurements from 19 locations from Finland. Also, an example scenario is given to show the effect of geographical spread on the aggregate power generation of multiple wind power generation units.

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

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.

Generating fault rate surfaces using network fault statistics and geographic information

This paper applies logistic regression to network fault statistics and geographic information. The logistic regression model is further processed in order to obtain raster formatted fault rate surfaces that aid network planning, e.g., in automated routines or as background maps. The fault rate surfaces are applied as a part of an automated network planning routine planning case based on real network data from the same distribution system operator as the fault data. The results can help in the planning of more reliable, yet economically feasible, power distribution networks.

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

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.

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

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.

Probabilistic prosumer node modeling for estimating planning parameters in distribution networks with renewable energy sources

With the increase in distributed generation, the demand-only nature of many secondary substation nodes in medium voltage networks is becoming a mix of temporally varying consumption and generation with significant stochastic components. Traditional planning, however, has often assumed that the maximum demands of all connected substations are fully coincident, and in cases where there is local generation, the conditions of maximum consumption and minimum generation, and maximum generation and minimum consumption are checked, again assuming unity coincidence. Statistical modelling is used in this paper to produce network solutions that optimize investment, running and interruption costs, assessed from a societal perspective. The decoupled utilization of expected consumption profiles and stochastic generation models enables a more detailed estimation of the driving parameters using the Monte Carlo simulation method. A planning algorithm that optimally places backup connections and three layers of switching has, for real-scale distribution networks, to make millions of iterations within iterations to form a solution, and therefore cannot computationally afford millions of parallel load flows in each iteration. The interface that decouples the full statistical modelling of the combinatorial challenge of prosumer nodes with such a planning algorithm is the main offering of this paper.

Reliability worth assessment of an advanced cable health monitoring scheme

Schemes that have been developed for cable fault detection and location can be categorized into two major groups, namely reactive fault diagnosis schemes (RFDS) and proactive fault diagnosis schemes (PFDS). RFDS identify the failed cable sections after a complete breakdown condition following the operation of protective devices, but PFDS can detect and locate the deteriorated cable sections while still in their incipient failure conditions. This paper aims to evaluate the impact of employing a typical PFDS on the major attributes of the service reliability. The study results are also compared against the reliability worth of employing a commonly used type of RFDS. The effect of the neutral treatment method in use in a power distribution system is also taken into account when directing the reliability case studies. The study results show that by employing PFDS in a power distribution system, it is possible to improve service reliability.