GREDUATION PROJECT
DESIGN OF A MICROGRID WITH ANALYZING ENERGY STORAGE DEVICES AND POWER QUALITY SPECIFACATIONS
Summary
In recent years, by increasing contaminating gas emissions and also deregulation of electricity market involves to usage of renewable energy sources extremely. Today wind, hydro and photovoltaic systems are the renewable energy sources that are most commonly used all around the world. Microgrid which became popular led to renewable energy sources include in power systems provides maximum energy usage. Because of the intermittent feature of the renewable energy sources there can be observed some important problems such as; stability, reliability and power quality. But when they are connected in a microgrid system they can overcome all of these problems.
When we look for the electric vehicles status, by developing in battery sector they are started to use widely. But with the increasing number of connected electrical vehicles to grid there will be some problems such as balancing, power quality and frequency. In this thesis structure of microgrid will be explained with analyzing storage units and also MATLAB simulation of a microgrid is designed by considering the electrical vehicle chargers with interconnected grid operations.
1) INTRODUCTION
a) Aim of the thesis:
The efficiency of the big power plants is up to 40% which means that they convert fuel to electricity up to 40%. But with the distributed energy resources (DER)s which include in microgrid, the efficiency increases up to 55%. By constructing these resources near the end users led to increase efficiency of electricity usage and also more reliable and healthy distribution system with microgrid operations. As some kind of electrical energy generation systems are used with microgrid such as micro hydro plants, micro diesel plants, wind turbine and photovoltaics etc. But the main idea for constructing microgrid in order to led the usage of renewable energy sources. Wind turbines and photovoltaics are most commonly used renewable energy sources in microgrid operations. In this thesis it is aimed that examining the feature of batteries that are used in microgrid integration and designing a microgrid which includes a wind turbine and also by using static synchronous compensator (STATCOM) in grid-connected microgrids is introduced in order to improve voltage sags and unbalances.
b) Literature Research:
Energy consumption is increasing day by day. By this increasing people are getting more conscious about the fuel expenditure and environment. Necessity of this conscious behavior is using present energy with most efficient way. In order to achieve this people are looking for new technologies and creating new solutions about this subject. Microgrid is one of the most important solution that gives the maximum energy usage and also lets the renewable energy sources integration to the grid with distributed way. Wind energy, photovoltaics and micro hydro plants are the most important clean technologies for electricity generation. When the structure of the microgrid is examined with distributed energy resources; high efficiency of energy usage, better quality of electric energy and low gas emissions make this operation one of the most important system that will be used more widely in near future. Also with the development of battery technologies microgrid operation & integration in power system supplies more reliable energy to the end users. When we consider all these advantages importance of microgrid operation can be observed exactly.
2) STRUCTURE OF MICROGRID
European Union has well-defined a Smart Grid as “ An electricity network that can cost efficiently integrate the behavior and actions of all users connected to it generators, consumers and those that do both in order to ensure economically efficient, sustainable power system with low losses and high levels of quality and security of supply and safety.” A new system which is also take in context of smart grid is called microgrid. Microgrid is a small grid which is generally connected low voltage levels between different type of distributed energy resources to the main grid with single point of connection (point of coupling, PCC). When we examine the main structure of microgrid; there are some micro sources such as wind turbines, photovoltaics, micro hydro plants etc. and also storage devices such as batteries, supercapacitors, flywheels etc. As distributed generators are used for generating electrical energy, energy storage devices are related with the compensation which are used for fulfilling the required demand because of the slow response of distributed generators. This system can be used for supplying electrical energy for different type of loads such as EV’s. And it can be installed with connected to grid mode and island mode. The important points that are given in the below should be considered while designing a microgrid.
• Detection of specifications about the distributed generators,
• Type of the generator that is suitable for the installation area (wind, solar or hydro etc.),
• Obtaining the power of the unit,
• Detecting the place of distributed energy resource in order to get maximum efficiency,
• Installing the best energy storage device in order to get reliable energy,
• Detecting the suitable compensator to increase controllability and transfer capability of the network,
• Obtaining the system requirements,
• Considering all power losses and also stability problems,
• Choosing one of the options if grid-connected mode or islanded mode.
Main structure of a microgrid is given in Figure 1.
Figure 1-Main Structure of a Microgrid
When the benefits of microgrid operation is examined it can be three specifications can be observed mainly such as flexibility of power system, power quality and power efficiency.
Flexibility: Flexibility is a feature of power system which is easiest to identify in the portfolio of power plants. The operations that occurs in microgrids makes the power network more efficient and more flexible. A smart microgrid can be controlled by different ways. As the demand can be arrange according to these operations a microgrid operation gives more flexibility to grid system and also electricity producer or end users which are include in electricity market.
Power quality: There are various methods in order to measure power quality and reliability of a system. Unscheduled outages are the big problems for end users as less dramatic effects occurs on power system such as voltage sags, swells, harmonics, imbalances etc. With constructing the microgrids near to end users give chance to get good quality of power and also provides reliability in short term or long term electricity usage.
Power efficiency: As the traditional energy generations are done out of the settlement the efficiency of power is decreasing while distributing. But with microgrid operations as distributed energy resources can be constructed near the residential areas the power systems get more efficient.
As it is shown on the Figure 1 Microgrid includes distributed energy resources, energy storage device, power electronic components and also different type of loads with interconnected system which has a point of coupling (PCC). Now all these structures will be discussed in 2.1, 2.2 and 2.3.
2.1) DISTRIBUTED ENERGY RESOURCES (DER)s IN MICROGRID
When the distributed energy resources for a microgrid operation is examined; micro diesel and combined plants can be given as example. Also micro hydro plants, wind turbines and photovoltaics are included in this operation as renewable energy sources. One of the main benefit of microgrid systems is giving chance to renewable energy resources integration. In this thesis it is aimed to design a microgrid system by using renewable energy resources. Before deciding the resource wind turbines, photovoltaics and micro hydro plants are examined.
2.1.1) WIND TURBINES
Wind is one of the renewable energy sources on the earth. Wind energy is being used for centuries for different applications. As the conventional sources for producing electrical energy that can not provide continuity meanwhile bringing environmental problems as the demand for energy increases, wind energy became very popular. Generally, to produce electricity some mechanical conversion has to take place. Since it is capable of moving substances, wind is very appropriate for the objective of obtaining energy. Wind turbines, which have a rotating part called rotor is forced to face the wind in various ways. When the rotor rotates, by using electrical generators, electrical energy is being produced. With this conversion of energy, no harm comes to the environment and continuity is perfectly achieved.
Wind turbines, are the modern relatives of the windmills. Centuries ago, windmills used wind energy to mill grains and pump water. With a similar physics, nowadays wind turbines are used to generate electricity by means of mechanical and electrical conversion. Wind energy is converted into mechanical energy by means of a rotating body, made up by blades and hub connected to a tower. After this process, mechanical energy is transmitted into the rotating part, rotor, of the generator in order to produce electricity.
When the power in the wind is examined the equation can be expressed as
is the mass over time and can be expressed in terms of : air density , : area the mass is passing through and : the velocity .
When these considerations are combined power in the wind can be expressed as
This equation has significant importance as it tells that power of the wind is dependent to the velocity of the wind and also the density of air.
Air density is assumed to be at and at a pressure of .
When the microgrid integration of wind turbines are examined; efficiency of these turbine is getting higher with these operations. Because of the intermittent specs of wind turbines by integrating in microgrid system stability, reliability and power quality problems are achieved with a success way. A microgrid structure which is islanded with a wind turbine is shown in figure 2.
Figure 2-Microgrid Structure with Wind Turbine in Islanded Mode
2.1.2) PHOTOVOLTAICS AND SOLAR HEATING TECHNOLOGY
Photovoltaics, or PV energy conversion directly converts the sun’s light into electricity. This mean that solar panels are only effective during daylight hours because storing electricity is not a particularly efficient process. Solar Energy has the greatest potential for providing clean, safe, and reliable power. The solar energy falling on the Earths continents is more than 200 times the total annual commercial energy currently being used by humans. Power of a solar system is directly related with the radiation of sunlight. Equivalent circuit of PV cells is shown in Figure-3 and according to day hours sun radiation and output power of the photovoltaic is shown Figure-4 and Figure-5.
Solar thermal electric energy generation which is different from photovoltaics concentrates the light from the sun to create heat. This heat is used to run a heat engine which turns a generator to make electricity. The working fluid that is heated by the concentrated sunlight can be liquid or gas. Different working fluids include water, oil, salts, air, nitrogen, helium etc. Different engine types include steam engines, gas turbines stirling engines etc. All of these engines can be quite efficient that is between 30% and 40%. Heat storage is a far easier and efficient method what make solar thermal so attractive for large-scale energy production. Heat can be stored during the day and then converted into the electricity at night. Solar thermal plants that have storage capacities can drastically improve both the economics and dispatch ability of solar electricity.
A microgrid can include both of these system. As the increasing demand side management of energy photovoltaics are started to use widely.
Figure 3-Equivalent Circuit of PV Cells
According to equivalent circuit I-V characteristic of photovoltaic cells is expressed as:
Where; IFV: Current produced by sun lights (A), Id: Diode current(A), Rp: Parallel resistance (Ω), Ip: Parallel resistance current (A), Rs: Serial resistor (Ω), Iy: Load current (A), V: PV battery output voltage (V), q : Electron charge (1,6×10-19 C), K: Bolztman constant (1.3806503 e-23 J/K), T: Cell temperature (K), I0: Diode saturation current (A).
Figure 4-Radiation of Sunlight
Figure 5-Output Power of PV
2.1.3) MICRO HYDRO PLANTS
In microgrid operations according to detection of most efficient location micro hydro
plants are used with interconnected systems. And also hybrid microgrid are preferred in
recent days in order to get maximum power and reliability. A hybrid system which has
been installed in Singapore is shown in figure-6.
Hydro power plants are used in order to generate electricity for many years. The main process of electricity producing with hydro power depends on the potential energy of the water. Stored water in dams flow from high level to low level. Flowed water hit the turbine which is going to turn the generator in order to produce electricity. The power that can be produced in a hydro power plants can be expressed as;
P = γ.H.Q.η (kW)
Where;
γ: The unit weight of water (9,81 kN/m3),
H: height (m),
Q: flow rate (m3/sn).
The efficiency of the power plant which have less than 100 kW power is 60-80%.
Efficiency of a hydro power plant depends on some factors such as; amount of water
which is stored, the height differences between the up and low level of the water that
means input and output location of water, direction of water correspond to the flood etc.
Figure 6-A Hybrid Microgrid System in Singapore
2.2) ENERGY STORAGE DEVICES IN MICROGRID OPERATIONS
Energy storage systems (ESS) have been using for a long time in different forms and applications. In recent years ESS is started to use for power systems such as smart grid and microgrid operations. Currently in many countries using of renewable energy sources is increasing day by day. But producing this renewable energy continuously is not available due to amount of wind, sun or tidal is not constant. This event gives non-dispatchable and intermittent feature to renewable energy sources. At this point energy storage systems become a such a solution of this problem. Storage devices can be connected to power systems at different points with generation, transmission, distribution or consumption levels. It can be separated to two main applications with usage of ESS. One is electrical distribution applications and other one is large scale grid applications. In Figure-7 it is shown that suitable energy storage devices that can be applied both of these applications.
Figure 7-Energy Storage Diagram
Important points which are given in the below explain the main benefits of ESS when they are used in microgrid systems that includes renewable energy sources.
• As renewable energy sources can not supply constant and stabile energy there are some problems that occur between load demand and energy sources such as unbalance situation. In order to maintain voltage and frequency of power system in a required range ESS are used in order to overcome this problem.
• They are used for power supply for preventing power outages and they create a reliable energy for power systems.
• ESS is also used for time shifting when the demand is high.
Depending on the usage are of the application energy storage devices are detected according to power system ratings and their own specifications. Usage are with rated power and discharge time levels are shown in Figure-8.
Figure-8 Technology Types of Energy Storage Devices with Differing Value Contributions
In a microgrid there can be integrated some types of ESS called as Distributed Energy Resources (DER). In the fallowing statements all these storage systems are explained in details.
2.2.1) Battery Energy Storage (BES)
Battery which is one of the oldest energy storage device that storage the electricity as electrochemical energy. Before they were generally used as lead acid and nickel cadmium but with the developing technology they are using with sodium sulfur and lithium-ion types. While charging and discharging batteries can provide fast response they can behave as a constant DC source. With a suitable power electronic component, they can provide four-quadrant operation like as bidirectional current flow and bidirectional voltage polarity. Key factors of batteries for storage applications include: high energy density, high energy capability, round trip efficiency, cycling capability, life span, and initial cost.
In microgrid operations batteries are the most popular and most cost-efficient devices for energy storage. Battery energy storage systems (BESS) correspond to series/parallel arrangements of individual batteries. They have been used for load leveling, spinning
reserve, power factor correction, and as a power surge module for load transient compensation.
2.2.2) Flywheel Energy Storage (FES)
Working principle of a flywheel energy storage system is storing energy in a rotating mass with the form of kinetic energy which is converted into electricity through a generator and electricity through a motor set. Moment of inertia of the rotating mass is proportional to energy stored in the system. Thus, by increasing the mass of the rotating system or rotational speed, energy storage capability can be increased. A flywheel rotates with a variable frequency so that a power electronic interface must be connected to machine into the grid. When the flywheel accelerates, energy is transferred from the grid into the flywheel and the electric machine acts as motor. Conversely, when the flywheel slows down, it acts as an energy source providing energy to the grid and the electric machine behaves as a generator.
Commercially flywheels are used in uninterruptible power supply (UPS) arena. They can supply continuous power for a few seconds after a loss of supply until backup generators are turned on. In a microgrid context, flywheels can be used as UPS systems, peak shaving in short time frames, load leveling, and for stability enhancement. Structure of a flywheel is shown in Figure-9.
Figure 9-Structure of Flywheel
2.2.3) Super Capacitor Energy Storage
Super capacitors also known as ultra capacitors which stores energy by collecting positive and negative charges separated by a dielectric material. Capacitance of the capacitor and square of the voltage applied across the device has a direct relation with the energy storage. Because of this relation the energy stored in capacitor can be boosted by increasing the capacitance or the voltage level. A super capacitor’s capacity ranges from several farads to tens of thousands farads and its power density is 10 times more than battery’s. When the other benefits of super capacitors examined;
• They have extremely high density up to 18 kW/kg that means in short period of time several hundred amps to several thousand amp of current can be released. So they are a suitable choose for short term power applications with high power operations.
• Fast charging and discharging feature make them storing energy without any chemical reaction. By using large big amount of current charging they can be recharged in tens of second although batteries need a few hours in order to complete charging and rapid charging.
• Also they have a long cycle life. The power that is supplied by super capacitors can be cycled for several hundred thousand times. When this feature of super capacitors is compared with the batteries they have more life cycle. Because batteries can be affected by some external factors such as; over-ripples, reverse polarity and deep discharges. They can withstand only some hundred up to a few thousand cycle.
• Working nominal temperature range of super capacitors is very high that is from -40 to 85℃. But this temperature range is only from -0 to 40℃ with batteries.
When these benefits of the super capacitors are considered they seem to be most preferable energy storage devices. But the price of the super capacitors is really so high. And also there are some other disadvantages such as;
• The electrolyte may leak with abnormal usage,
• With high frequency charging and discharging levels they are not suitable for usage, because the high frequency of rapid charging and discharging can lead to internal heat, decay of capacity and internal resistance increased, in some cases even would cause the collapse of the capacitor’s performance
• They cannot series connected because of the voltage balancing problems of monomers, and also serial connection may cause capacitors over voltages that give damage to capacitors and affect the performance
As it is mentioned before microgrid has two operation mode. One is grid-connected mode when the microgrid operates parallel to the distribtion network and other one is island mode where there is no grid connection. Super capacitor usage in terms of microgrid operation plays important role because the bulk grid failure the requirements are satisfied by the super capacitors. They will help to smooth transition with both of these two operation mode. Basic configuration of a super capacitor energy storage (SCES) in migrogrid operation is shown in Figure-10.
Figure 10- Basic Configuration of SCES in Microgrid System
2.2.4) Superconducting Magnetic Energy Storage (SMES)
Superconducting magnetic energy storage systems in form of magnetic field. The energy which is stored in this system is proportional to the inductance of the coil and the square of the circulating current. Resistive lose which is occur in circulating current through the superconducting coil can be neglected. So that by increasing inductance of the coil or circulation current capability of the stored energy can be increased. They have fast response and