Grid capacity issues with distributed generation A …

1 1 grid capacity issues with distributed generationA German case studyNofel Dakhel, Olof BohgardAbstract The purpose of the project was to investigate howa high penetration of solar power (PV) affects the electrical gridon a distribution level concerning active power and investigatedifferent measures for avoiding overloads and overvoltages, usinga simulation model and grid data received from Thestudied measures are battery storages of different technologies,load shifting and biomass power simulations weredone in the simulation tool DIgSILENT results showed that implementation of battery storage andload shifting will balance the grid and less power will be deliveredto or taken from the transmission grid . Load shifting is very hardto analyze and utilize but assumed to have low capital shifting in households is also a very immature energy is the most effective measure for balancing theactive power because of the valuable property to store energyand use when it is needed.

2 But the costs of battery storages arehigh even if no costs of power electronics were included and alsothe total energy losses will be higher. Implementation of a 10MW biomass power plant will balance the active power whilelow production of PV and high Terms Power systems, energy storage, PV, load INTRODUCTIONCLIMATE change is more evident than ever as reflected inThe European Union environmental directives which saythat the carbon emission must be reduced by 20% and 20%of the used energy must come from renewable energy sourcesuntil 2020. German government has also made a decision toclose the nuclear power plants until 2022 and in long termreplace them by the installed PV capacity is not homogeneously spreadacross Germany, some regions are already experiencing a veryhigh PV penetration of more than 200 kWpkm2comparedto the nationwide average of 39 kWpkm2.

3 In these regions,the increasing power feed-in may result to high local voltagemagnitudes which might give rise to demanding grid rein-forcement actions, causing in an increment of the whole PVgrid integration expenses in Germany [1]. One of the biggestchallenges in the reconstruction to an energy system with alarger part of renewable energy is the expansion of both thedistribution grid and transmission grid . To be able to transferlarge amount of energy from regions with energy excess toregions with deficit, large investments are needed to enhancethe transmission grid capacity MEASURESA. Energy StorageEnergy storage means an energy conversion process thatconverts different forms of energy ( chemical, thermal,mechanical energy) into storing forms of different energy storages, when needed, the stored energy can beconverted in electrical energy and fed in to the ) Li-ion Battery Storage:Li-ion batteries are primarilyused as medium-term (a few hours) storage, but can alsobe used as short-term (a few minutes) storage.

4 The Li-ionbatteries are the most important battery storage technology inthe portable application area (cell-phones, laptops). In the US,the Li-ion battery storage is already used in some areas wherethe grid is very weak [2].2) Lead-Acid Battery Storage:Its mainly used in short-term (a few minutes) and medium-term (a few hours) energystorage applications. The lead-acid battery technology has thelargest installed capacities all over the world (mostly due tocar batteries) and many existing have been in operation for upto 20 years [2].3) Vanadium Redox flow battery storage:The active mate-rial in the redox flow batteries is made up of salt and dissolvedin a fluid electrolyte which is stored in tanks. In vanadiumRedox flow batteries, the electrolytes are based on higher required energy density is the bigger the size of thetank needs to be.

5 The main advantage of the Vanadium RedoxFlow battery is its independent scaling of power density andthis type of batteries offers a big potential for relatively cheapweekly storage [2].B. Load ShiftingThe process of shifting loads from peak periods to off-peak periods is called load shifting. The main purpose forthe customer is to take advantage of the low electricity pricein the off-peak hours when using heavy load consumptionunits. Load shifting can be implemented in a number of example using Demand Response Programs which shiftsloads by controlling the functions of refrigerators, waterheaters and air conditions at peak hours. Energy storage is alsoan important feature of load shifting, for example pumpedhydro facilities pumps water from low reservoir into higherone during off-peak hours, and then reverses the flow duringpeak hours to generate electricity.

6 In the simulation modelit is assumed that 50 % of the households have one of thecontrollable loads available for Biomass Power PlantBiomass power plants generate carbon neutral1electric-ity from renewable organic waste that would otherwise bedumped. The generated energy is neutral is a term used to describe fuels that neither contribute tonor reduce the amount of carbon dioxide into the atmosphere [3]2 Its unlike the other renewable energy sources like wind andsolar which only produces electricity when the wind is blowingor the sun is shining. The biomass power plant implementedin the simulations i producing 10 MW SIMULATIONSThe goal of the simulations is to with help of implementedmeasures be able to not exceed the voltage limits set bygrid codes.

7 The grid model is medium-voltage to low-voltagemodel. In the model three of the low-voltage grids havedetailed models, where every household is represented bya load. The load for the low voltage grids that are onlyrepresented as load or a load combined with a PV generatorhas been dimensioned after the size of the transformer thatconnect the low-voltage area to the grid model is treated as an isolated medium voltage gridwith only one connection point to the transmission grid . Theconnection contains two transformers with a transfer capacityof 75 scenarios have been investigated. Scenario 1 describesthe current situation in the area and scenario 2 describes afuture situation where the PV installation in the area haveincreased with 30%.

8 The increased PV penetration has beendone in the low voltage areas with low production from PVFor every scenario worst case load and production profileshave been chosen. The most interesting worst case scenariosare High load & Low Production and Low load & RESULTSB attery nr kW11302125330041205120670750870930106011 20125013701430151100 TABLE IINSTALLED BATTERIES IN THE grid TO AVOID OVERLOADINGS ONTRANSFORMERS AND CABLES. THE BATTERIES WERE DIMENSIONEDFROM THEHIGHPRODUCTION& LOWLOAD 1 shows the loading of the transformers in the gridmodel between low voltage areas and medium voltage areas(secondary substation) for the High production & Low loadprofile. 14 of the transformers have a loading over 100 %for the base case, as the batteries are dimensioned to preventoverloading all the overloaded transformers are reduced bythe battery implementation.

9 The load shifting implementationreduces 7 of the overloaded transformers. Some of voltages ingrid are higher than 10 % above nominal voltage. They areFig. 1. The loading of the transformers for base case, battery and load located in the low voltage areas and are reduced by boththe battery and load shifting implementation. Figure 2 showsFig. 2. The loading of the transformers for with batteries loading of the transformers at secondary substation in thegrid for the High production & High Load profile. Despitethe high load, five transformers are still overloaded. Theyare all reduced by the battery implementation. The batteriesreduce the loading of the transformers such that none are moreloaded than 70% because all the batteries are charging withfull figures shows a 24 hour simulation with the basecase (red) and a case with a combination of all the measuresused (blue).

10 Figure 3 is a sunny day and figure 4 is for acloudy winter day. In the figure 3 the arrows illustrate whichmeasures that create the difference in power delivery betweenthe base case and the mixed combination case are used. ForFig. 3. 24 hour simulation for a sunny day, base case red and combinationof different measures 4. 24 hour simulation for a winter day, base case red and combinationof different measures summer day there are no bigger differences between thetwo graphs. But it is enough to prevent the transformers tonot be overloaded. The load shifting also reduces the highestproduction peaks. The simulation also shown that transformerswere overloaded for long time during a day with a lot of PVproduction, even if the loads were hourly CONCLUSIONL arge integration of PV into the distribution network willlead to high voltages and overloading, especially in the low-voltage side and the secondary station.