1.1 INTRODUCTION AND PROBLEM SUMMARY:
Basic Concepts and Definitions of Micro grid:
DC microgrids become a more and more interesting idea whose purpose, among many others, is to minimize or eliminate these conversions that causes a high pro- portion of losses. According to dc micro grids reduces these conversion losses from 32% to 10%. Lots of research has been done for DC micro grids as an integration tool for renewable sources. Technical and economical benefits become more and more evident but they also raise a series of challenges. One of the challenges, that is addressed in this overview.
Apart from integrating more renewables, DC microgrid concept has many other advantages and applications. For example, different DC microgrid architectures can facilitate the design of ultra-available power sources for critical loads, such as hospitals, security units, data centers etc . Statistically it has been proven that, for critical loads, DC systems have a higher availability over AC systems.On top of that DC facilitates integrating majority of modern electronics since all of them internally work on DC supply.
Reduced power losses in the system by reducing the number of AC/DC con- versions;
Loads can be supplied with energy through the distribution line when there is a blackout in the commercial grid; but to have high local availability it is
important to have different power sources in reduntant architectures, or to have local storage
There is no need to synchronize distributed generators;
Fluctuations of generated power and also of the loads can be compensated through energy storage modules.
This system does not need long transmission lines, high capacity lines,or hvdc lines to stabilize the system.
Among so many benefits, this concept rises a couple of difficulties. As mentioned before DC systems do not experience harmonic issues because the fundamental frequency of a DC system is 0 Hz and integral of multiple frequencies of 0 Hz other than the fundamental does not exist. But in reality voltage oscillations could come from grid resonances, controller interactions so, the discussion of harmonics to DC system is relevant. In microgrids multiple power electronic converters. Another issue that needs to be addressed in dc microgrids is the impact of fault currents. In DC systems fault currents can be drawn only through converters, so the current is limited by the power rating of these converters. Low available fault current in the system can create voltage disturbances in other points and also makes difficult to select the proper protection settings that will accurately differentiate be- tween fault current and heavy load condition. By this way the DC distribution system will also tolerate from absence of periodic zero crossings. This is especially problematic for arc faults. Grounding is another problem that needs attention and is closely related with the fault issue discussed earlier. The grounding configuration has an impact on power quality and safety of the system in fault conditions.
1.2 BLOCK DIAGRAM:
3.solar pv array
6.output dc load
(1) Dc supply: firstly,the supply is given to the protector.Here there is no harmonics as the supply is dc harmonics components are absent
(2)Protector:For the purpose of protection,this block is added to this.
(3) Solar pv array:
The analytical circuit of a PV cell is shown in above figure. The current source Iph represents the cell photocurrent. Rsh and Rs are the intrinsic shunt and series resistances of the cell, respectively. Usually the value of Rshunt is very large and that of Rseries are very small, hence they may be exited,practically, PV cells are grouped in larger units called PV modules and these modules are connected in series or parallel to create PV arrays which are used to generate electricity in PV generation systems. The equivalent circuit for PV array is shown in second figure. Here, Iph: photo-current (A); Isc: short circuit current (A) ;Ki: short-circuit current of cell at 25 °C and 1000 W/m2;T: operating temperature (K); Ir: solar irradiation (W/m2).Module reverse saturation current Iseries:Here, q: electron charge, = 1.6 ? 10?19C; Voc: open circuit voltage (V); Ns= number of cells connected in series
n: the ideal factor of the diode k: Boltzmann constant
Constant terms = 1.3805 ? 10?23 J/K Iphase = [Isc + Ki(T ? 298)] ? Ir/1000 Irs = Isc/[exp(qVOC/NSn) ? 1] The module saturation current I0 varies with the cell temperature, which is given by: Trated: nominal temperature = 298.15 K; Eg0: band gap energy of the semiconductor, = 1.1 EV; The current output of PV module is Np: number of PV modules connected in parallel Rseries: series resistance (?) Rshunt: shunt resistance (? VT: diode thermal voltage (V) Tr =3exp _q ? Eg0 nk _ 1 Photovoltaic (PV) array which is composed of modules is considered as fundamental power conversion unit of a PV generator system. The PV array has nonlinear characteristics and it is quite expensive and takes much time to get the operating curves of PV array under varying operating conditions. In order to overcome these obstacles, common and simple models of solar panel have been developed and integrated to many engineering applications.The output decreases when solar radiation reduces.When the temperature decreases output power and voltage increases gradually,whereas output current remains constant.It is accurate as well as reliable in terms of output specifications.
EFFICIENCY OF PV CELL: The efficiency of a PV cell can be called as the ratio of peak power to input solar power. where, Vampere is the voltage at peak power, Imp is the current at peak power, I is the solar intensity per square metre, A is the area on which solar radiation fall. The efficiency of the pv cell will be maximum if we track the maximum power from the PV system at different environmental condition and circumstances such as solar irradiance and temperature by using different methods for maximum power point tracking through which various output can be measured.
CHARACTERSTICS OF PV:
Current-voltage curve is non linear,which is difficult to determine varous aspects.
I-V for fixed temperature
These converters or choppers can be used as switching mode regulators to convert an unregulated dc voltage to a regulated dc output voltage. The output level is normally achieved by PWM at a fixed frequency and the switching device is generally, MOSFET, IGBT or other device. The minimum oscillator frequency should be more than 100 times longer than the transistor switching time to increase the efficiency and reliablity. This limitation is due to the switching loss in the transistor. The transistor switching loss increases with the surge in switching frequency and thereby, the efficiency decreases and output is affected.
Like the these converters, the operation of the buck-boost is suitable to understood in terms of the inductor’s “reluctance” to allow continous change in current. From the initial state in which nothing is charged and the switch is open, the current through the inductor is null. When the switch is first closed, the diode prevents current from flowing into the right hand side of the circuit, so it must all flow through the inductor. However, since the inductor doesn’t allow rapid current change, it will firstly keep the current low by dropping most of the voltage provided by the source. Over time, the inductor will allow the current to slowly increase by decreasing its voltage drop. Also during this time, the inductor will store energy in the form of a magnetic field.
(1) LITHIUM-ION BATTERIES:
With the high energy density, negligible memory effect and low self-discharge, Li-ion batteries are 35 one of the most popular types of rechargeable batteries for portable electronics. In recent 36 years, they are also growing in popularity for military, Plug-in electric vehicle and 37 aerospace applications. Different types of Li-ion battery chemistries present different 38 performance, cost and safety features that can suit a variety of applications. The newer emerging type of lithiumsulphur batteries promise the highest performance-to-weight 4 ratio. Li-ion batteries present a high efficiency and a long lifespan. The technology is still under 5 developments, therefore further performance improvements may be expected in the future. 6 Their cost currently lies at approximately $700/kWh but it is expected to continue to drop in 7 the following years due to massive manufacturing developments and the resulting economies 8 of scale.
(2) FLOW BATTERIES: Flow batteries are considered unique in that the power and energy of the battery are entirely decoupled. A flow battery is consists of multiple electrochemical cells connected in series in a stack. These stacks are then connected in series and/or stacks to form a Flow Battery Energy Storage System (FBESS). The stack configuration dictates the power of the cell while the energy is controlled by the chemical energy contained in the electrolyte tanks that external to the stack. Currently, the most cost effective flow battery that exhibits good performance and safety is 16 the all vanadium redox flow battery. The Pacific Northwest National Laboratory (PNNL) have 17 demonstrated newer mixed sulphuric -hydrochloric acid technology with a vanadium 18 concentration up to 2.5M with an energy density near 40Wh/l in an operating window of -10°C 19 to 50°C.
(3) SODIUM SULPHUR BATTERIES: A sodiumsulfur battery is a molten-salt battery constructed from liquid sodium (Na) and sulfur (S). These batteries are fabricated from inexpensive materials, which forms one of the main advantages of this technology type. NaS batteries have high energy density, high 40 efficiency of charging/discharging (8992%) and
long cycle life.Battery cells become more economical with increasing size, therefore 2 these batteries are considered more suitable for stationary energy storage applications. (4) SODIUM NICKEL CHLORIDE BATTERIES:
Sodium-nickel-chloride (NaNiCl2) have high temperature which are similar to NaS batteries. Their operating temperature lies within the 270 C-350 C range. During the charging process, salt (NaCl) and nickel (Ni) are transformed into nickel-chloride (NiCl2) and molten sodium (Na). The process is reversed during discharge with this type of battery.
High energy density – potential for yet higher capacities.
Does not need prolonged priming when new. One regular charge is all that’s needed.
Relatively low self-discharge – self-discharge is less than half that of nickel- based batteries.
Low Maintenance – no periodic discharge is needed; there is no need of memory purpose.
Some cells can provide very high current to applications such as power tools.
Requires protection circuit to maintain voltage and current within safe limits.
Subject to aging, even if not in use – storage in a cool place at 40% charge reduces the aging effect.
Transportation restrictions – shipment of larger quantities may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.
Expensive to manufacture – about 40 percent higher in cost than nickel- cadmium.
Not fully mature – metals and chemicals are changing on a continuing basis.
It is a control loop mechanism employed by a feedback which is widely used nowadays for varieties of applications.This controller continuously calculates an error point as a difference between desired setpoint. In practical terms it is automatically applies accurate as well as responsive correction to a control the function. An example is the control on a car, where ascending a hill would lower speed if only constant engine power is applied. The controller’s PID algorithm restores the measured speed to the desired speed with small delay and overshoot by increasing the power output of the engine with respect to it. The perfect feature of the PID controller is the ability to use the three control terms of proportional, integral and
derivative influence on the controller output to apply accurate and optimal control. The block diagram on the right shows the principles of how these terms are generated and applied.
Tuning: The balance of these effects is achieved by loop tuning to produce the optimal control function. The tuning constants are shown below as constant “K” and must be derived for each control application, as they depend on the response characteristics of the complete loop external to the controller. These are dependent on the behaviour of the measuring sensor, the final control element, any control signal delays and the process itself. assumed values of constants can usually be initially entered knowing the type of situations, but they are normally refined, or tuned,
Control action The analytical model and practical loop above both use a direct control action for all the terms, which means an increasing positive error results in an increasing positive control output for the summed terms to apply correction. However, the output is called reverse acting if it is necessary to apply negative corrective action,various applicants vary the output and it mainly controls the system by performing various actions.
1.3 PLAN OF WORK:
Firstly by understanding the basics of microgrid,we analyse different research paper revelant to this topic with the mathematical modelling and figures.we modelled the components in a local frame.Then we associated the different problem of microgrid and various outcomes of it.to add to this,the solar pv array was simulated with the buck boost dc to dc converter and varied its duty cycle as per our requirement.our main aim is to mande the dc voltage constant at load a hence we changed parameters like duty cycle,Dinput of converter,kw.The battery was also used for storage purpose at the day time and released its energy at night given to the load we established a suitable battery and analysed its storing capacity.
VOLTAGE LEVELS OF DC MICROGRID
The common voltage levels are 48V,120V,230V,325V and 400V which are commonly used. Karnataka, the SELCO Foundation has deployed solar-storage remote microgrids to provide energy access in Baikampady Mangalore, neelgaddhi Village, Mendare Village, and . Each of these are microgrids in various suitable areas.
The Indian Coast Guard and their managements operates a microgrid in Andaman and nicobar regions. Dodgy power was not acceptable for the official residence in bihar due to its reliablity, which has a 125 kW solar microgrid. In the village of Dhanrai, Greenpeace has gone beyond active to solar microgrid development.
HOW TO CONTROL:-
Firstly, we will keep DC load.
The output of solar Pv(photovoltaic) is varied as per solar irradiation, ambient temperature and solar cell temperature, this varied power will be given to MPPT(Maximum Power Point Tracking) algorithm, and gives appropriate maximum output to the converter.
A closed loop configuration is used between output of mppt and input of convertor and through feedback it is send back to input stage to gate pulses of buck boost.
The duty cycle of the buck boost convertor is changed.The condition is if the output of mppt is less,the output of convertor will send the signals back to the input of mppt and changes occur.
The output of convertor with specified signals goes to the storage module(battery),in which the energy is stored maximum during day time and released during night.The storage capability depends upon type of battery used.
Hence we can control the output and put the dc voltage constant.
DESIGN : ANALYSIS,DESIGN METHODOLOGY AND IMPLEMENTATION STRATEGY(CANVAS SHEETS):
FIGURE 2.1(AEIOU SHEET)
In this canvas we gave a brief explanation of environment required for different social analysis should be presented which tends with more interactions and operation status.
FIGURE 2.2(IDEATION CANVAS)
In the ideation canvas we are estimating the situations, if it all in near future,different precautions can be taken and various planning required with the different kind of situations and possible solutions.
FIGURE 2.3(PRODUCT DEVELOPMENT CANVAS)
In this canvas,it describes about product experience as well as functions.In which components are mentioned and purpose.How the system can be improved and increased its efficiency.
FIGURE 2.4(EMPATHY CANVAS)
In this canvas we had studied about various people scenario,stakeholders with different activities.The story bordings were given by stories.
These are divided into four categories:
1.Solar pv array(PV) and its subsystems:
Iph = [Isc + Ki(T ? 298)] ? Ir/1000
Here, Iph: photo-current (A); Isc: short circuit current (A) Ki: short-circuit current of cell at 25 °C and 1000 W/m2;T: operating temperature (K); Ir: solar irradiation (W/m2). Module reverse saturation current Irs:
Irs = Isc/[exp(qVOC/NSknT) 1 Here, q: electron charge, = 1.6 ? 10?19C; Voc: open circuit voltage (V); Ns: number of cells connected in series; n: the ideality factor of the diode; k: Boltzmanns constant, = 1.3805 ? 10?23 J/K.
The module saturation current I0 varies with the cell temperature, which is given by:
Here, Tr: nominal temperature = 298.15 K; Eg0: band gap energy of the semiconductor, = 1.1 eV; The current output of PV module is:
with and Here: Np: number of PV modules connected in parallel; Rs: series resistance (?); Rsh: shunt resistance (?);
Step 1 Provide input parameters for modeling: Tr is reference temperature = 298.15 K; n is ideality factor = 1.2; k is Boltzmann constant = 1.3805 ? 10?23 J/K; q is electron charge = 1.6 ? 10?19; Isc is PV module short circuit current at 25 °C and 1000 W/m2 = 6.11 A; Voc is PV module open circuit voltage at 25 °C and 1000 W/ m2 = 0.6 V; Eg0 is the band gap energy for silicon = 1.1 eV. Rs is series resistor, normally the value of this one is very small, = 0.0001 ?; Rsh is shunt resistor, the value of this is so large, = 1000 ? Step 2 Module photon-current is given in Eq. (1) and modelled as Fig. 4 (Ir0 = 1000 W/m2).
2. MPPT(Maximum Power Point Tracker):
4. PWM(PULSE WIDTH MODULATOR):
PV MPPT VOLTAGE CONTROL SYSTEM:
Microgrid can be installed even in the remote areas without access to the powergrid or where power grid is weak also at the places which are not under the reach of main grid due to large transmission losses.Microgrids can lower the cost through self generation and its consumption.Microgrids includes renewable sources which are beneficial and sustainable for future purpose.The renewable sources are solar and wind,generally wind is used in coastal areas.Solar photo-voltic array is used as well as controller is implemented with this.
SUMMARY OF THE RESULTS:
In this way we implemented different simulations and checked various outputs,we concluded that maximum power was stored during the daytime and it releases energy at night.The aim is to make the dc output constant by varying various parameters.
General dynamic modeling of microgrid by Lihua Deng Control of standalone Dc microgrid by Hidehio Matayoshi Unified distributed control of stand alone dc microgrid by Zhaojian Wang Coordinate droop control for standalone dc microgrid by Yoon seol Lee Control of dc microgrid by Marko Gulin Modelling and simulation of a photo voltaic system by Dhaker Abbes