Microgeneration, Energy Storage, Power Converters and the Regulation of Voltage and Frequency in the Low Voltage Grid
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1 Microgeneration, Energy Storage, Power Converters and the regulation of voltage and frequency in the Low Voltage Grid Manuel Campos Nunes, Instituto Superior Técnico, Universidade de Lisboa. in order to reduce the emission of greenhouse gases, focusing Abstract— This paper focuses on the study and development of on the other hand on sustainable energy and energy efficiency a system composed by Microgeneration (MG) and energy storage (e.g Kyoto Protocol or EU2020 [1]). (ES), which together with other similar systems, might avoid over- Many countries offered big incentives for renewable energy and undervoltages, as well as mitigate voltage dips. The system generation. Consequently MG, most of the times using RES, may use the stored energy on deferred. These systems are expected to contribute to the regulation of the voltage and frequency of a has become increasingly popular and distributed generation low voltage grid, especially in the case of an isolated grid. (DG) is part of the electrical grid nowadays. However despite The increase of distributed generation, using mainly renewable all benefits of DG (economical, environmental, reduce of losses energy sources (RES), motivated some technical and operational etc.), it motivates some technical issues and contributes to the issues that have to be approached. Despite all the benefits of deterioration of electric power quality. The electric power distributed generation and renewable energy, the intermittent system changed a lot with the introduction of DG, which character of such type of source and the mismatch between supply and demand might lead to some problems in reliability and consequently leads to a new paradigm with bidirectional power stability of the grid, and to an inefficient use of RES. flow. The integration of energy storage systems (ESS) together with The use of ESS in parallel with Microgeneration can help to MG can help to achieve a better integration of RES, contributing mitigate some electric power quality issues and even improve to the reliability and stability of the grid. Besides that, the the integration of RES into the grid [4]. These systems might be decentralized action of these systems can help adjust the voltage beneficial for the consumers and for the grid operator. and frequency, contributing to the improvement of the electric According to the recently approved decree law in 2014 by the power quality when the values go beyond the limits imposed by the norm NP EN 50160. Portuguese government [5] (Decreto-Lei nº 153/2014) the The model of a system composed by Microgeneration and produced energy through MG should serve the effective energy storage was developed using the Matlab/Simulink necessities of the installation to which it is associated. An ESS software, as well as one rural low voltage grid. The capacity of installed in parallel to the MG is therefore highly benefic to the these systems to regulate voltage and frequency was simulated, consumers. For example, in case of photovoltaic energy there is analyzing the values of active and reactive power, the voltage level a huge mismatch between supply and demand [6]. Besides all profile, as well as the frequency. benefits for the consumer, these systems can also be very Index Terms— Distributed generation, Microgeneration, advantageous to the grid operator [6], one of them being the Energy Storage, Electric Power Quality, Frequency and Voltage improvement of electric power quality. This way, the impact of Regulation MG systems into the grid might be reduced, meaning that through the decentralized action of several of those systems the voltage and frequency of the grid could be regulated. I. INTRODUCTION Considering the above, it is intended this way through MG HE economic and social development in many developed and ES systems to avoid overvoltages and undervoltages, as T and developing countries is contributing to the growth of well as to mitigate voltage dips. These systems are expected to energy consumption in those countries. It is expected that this contribute to the regulation of voltage and frequency of a low tendency remains for the next decades, although efforts are voltage grid, especially in the case of an isolated grid. being made to improve energy efficiency [1]. According to [2], it is expected a 40% increase in world energy consumption by II. MODEL OF THE LOW VOLTAGE GRID 2040 in comparison to 2012. A rural low voltage grid was modeled using the software Almost 75% of the energy consumed nowadays is obtained Matlab/Simulink in order to study the impact of the MG with through fossil fuels [3]. However due to economic and ES systems on it. The main goal was to replicate a typical rural environmental issues, the interest in renewable energy has grid and therefore all components of the grid (Medium Voltage increased in the last years. There have been many agreements (MV) to Low Voltage (LV) Transformer, Electrical Lines and 2 Loads) were scoped according to the rules of low voltage grid of houses: installations [7], as elaborated in [8]. TABLE I The rated value of the voltage in the MV part of the grid is DIVERSITY FACTOR BY THE NUMBER OF HOUSES [6] 30kV, while in the LV part this value is 400V (line voltage). 10- 15- 20- 25- 30- 35- 40- N 2-4 5-9 ≥50 The connection of the medium to the low voltage is made by a 14 19 24 29 34 39 49 630kVA transformer. C 1,00 0,75 0,56 0,48 0,43 0,40 0,38 0,37 0,36 0,34 This way, the power consumed by each equivalent load and in the first branch of the distribution network is: TABLE II POWER CONSUMED BY EACH LOAD IN THE FIRST BRANCH Installed Actual power Equivalent Number of Diversity power supply required Load houses Factor [kVA] (3 phase) [kVA] 1 8 55,2 0,36 19,87 2 10 69 0,38 26,22 3 8 55,2 0,43 23,74 4 5 34,5 0,56 19,32 5 2 13,8 0,75 10,35 6 3 20,7 0,75 15,53 Fig. 1. Model of the low voltage rural grid 7 2 13,8 1 13,80 8 2 13,8 1 13,80 As it can be seen in Fig. 1 there are four branches in the LV 1st Branch 40 276 --- 142,62 part of the grid. The first branch (R1), which will be the focus of this paper, represents a collection of households, while the It can be seen that the apparent-power supply of each load second (RR1) and third branches (RR2) are some other rural must take into consideration the diversity factor, multiplying it grid networks. The fourth branch characterizes a local network by the installed power. (RL). The power assigned to the first branch is 200kVA as well For the other branches and since their loads are only as to the RR1 rural network, while the other branches receive represented by an equivalent load each, the load factor is 100kVA each. assumed to be 0,75. Due to computational reasons, each load of the first branch TABLE III represents a group of houses. In total there are 40 singular POWER CONSUMED BY EACH EQUIVALENT LOAD OF BRANCH 2, 3 AND 4 households, distributed by each one of the 8 equivalent loads. Each household consumption is assumed to be 6,9kVA [7]. The Installed Load Actual power supply Branch transmission of electrical energy is made by overhead power power [kVA] Factor required (3 phase) [kVA] lines and the distance between the last equivalent load and the 2 – RR1 200 0,75 150 substation is 514m, which is a reasonable value for this 3 - RR2 100 0,75 75 topology of grid. 4 - RL 100 0,75 75 Some systems, composed of MG and ES, are also placed in the modeled grid, and they represent some of the structures The power to be consumed by the 4 branches should be placed in consumer households. Like the equivalent loads, these 437,6kVA which corresponds to a high load scenario. In order groups might represent more than one MG with ES system, to size the electric lines, the power flowing in each line has to depending on the number of systems attributed to each be taken into consideration. The current might be calculated household. using equation (1), being S the apparent power supply required and Un the nominal voltage: A. Distribution network sizing 푆 퐼푠 = ( 1) The dimensioning of all electric cables was made, √3푈푛 considering the power consumed in each branch of the low voltage network. As mentioned before, all loads pictured in Hence, the maximum current transported in each branch can Fig.1 represent a group of houses. However, not all houses be calculated. The results are shown in Table IV: consume maximum power simultaneously. Therefore in order to proceed to the sizing of the cables, the power of such loads was multiplied by a factor that stands for the simultaneous maximum demand of many houses (diversity factor), downstream the point that is considered. The value of this factor C, can be seen in the following table considering N, the number 3 TABLE IV POWER CONSUMED BY EACH EQUIVALENT LOAD OF BRANCH 2, 3 AND 4 Transited Maximum Branch power [kVA] current [A] 1 – R1 142,623 205,86 2 – RR1 145 209,23 3 – RR2 75 205,86 4 – RL 75 205,86 Fig. 2. Model of Microgeneration with energy storage system (ESS) Keeping in mind that the maximum voltage drop in a LV network branch defined by the Portuguese distribution operator The connection of the system to the grid is made by an EDP is 8%, the following type of overhead power lines was inverter, converting DC waveforms into AC ones.