CHAPTER – 1
Modern day power systems are complicated networks with number of generating stations and load centers being interconnected through power transmission lines. An electric power system has three separate components – power generation, power transmission and power distribution. Electric power is generated by synchronous alternators that are usually driven either by steam or hydro turbines. Depending upon the type of fuel used for the generation of electric power, the generating stations are categorized as thermal, hydro, nuclear etc. Many of these generating stations are remotely located. Hence the electric power generated at any such station has to be transmitted over a long distance to load centers that are usually cities or towns. Besides that, fossil fuel price is fluctuating due to the global economic and limited resource, it found that producing electricity with conventional fossil fuel will lead to the environment pollution Moreover, the modern power system is interconnected, i.e., various generating stations are connected together through transmission lines and switching stations.
Throughout the last century and in the present, the size of generating plants has been increasing. A new trend however is emerging currently, in which significantly smaller sized generating units are being connected at the distribution level. Some of the factors that contribute to this trend are listed below.
– Greenhouse gas issues have become very significant in many countries to consider dispersed energy sources such as solar, wind and wave that operate with smaller sized units.
– Local generating units using gas micro turbines are becoming more economical when the transmission and distribution overheads are taken into account. Fuel cells are still more expensive but have been showing great promise for low cost reliable small size generation units.
– Even though solar cells currently require a very large area and substantial investments, they can be used for power in large office buildings during business hours.
– The move to open competitive markets in electricity has increased the uncertainties of supply. In response to this uncertainty of central supply there was a massive increase in demand for back-up generation with the possibility of generation back into the grid when conditions suited.
Interconnection of small, modular generation to low voltage distribution systems can form a new type of power system, the microgrid. Microgrids can be operated connected to the main power network or autonomously, similar to power systems of physical islands, in a controlled, coordinated way.
1.2.1 Technical, economic and environmental benefits
Minimization of the overall energy consumption
Improved environmental impact
Improvement of energy system reliability and resilience
Cost efficient electricity infrastructure replacement strategies
Cost benefit assessment
1.2.2 Technical challenges for Microgrids
Relatively large imbalances between load and generation to be managed
Presence of power electronic interfaces
Protection and safety
Specific network characteristics
Use of different generation technologies
Electric Power Quality
The main concern of consumer side of electricity was the reliability of supply. Reliability refers to continuity of electric supply, even though the power generation in most countries is fairly reliable, the distribution is not always so. It is however not only reliability matters for the consumers, but also quality too is very important to them. For example, a consumer connected to the same bus that supplies a large motor load may have to face a severe dip in his supply voltage every time the motor load is switched on. In some extreme cases, consumers have to bear with blackouts. This can be quite unacceptable to most customers. There are also very sensitive loads in the system such as hospitals (life support, operation theatre, and patient database system), processing plants (semiconductor, food, rayon and fabrics), air traffic control, financial institutions and numerous others that require clean and uninterrupted power. In several processes such as semiconductor manufacturing or food processing plants, a batch of product can be ruined by a voltage dip of very short duration. Such customers are very largely affected by such dips since each such interruption cost them a substantial amount of money. Even short dips are sufficient to cause contactors on motor drives to fail. Stoppage in a portion of a process can destroy the conditions for quality control of the product and require restarting of production again. Thus in this changed scenario in which the customers increasingly demand quality power, the term power quality (PQ) attains increased significant important.
The most common power quality problems in the system are:
Power frequency variations.
Voltage sag : A voltage sag as defined by IEEE Standard 1159-1995, IEEE Recommended practice for Monitoring electric power quality, A sag is a decrease to between 0.1 and 0.9 pu in RMS voltage at the power frequency for durations from 0.5 cycle to 1 min. The measurement of a voltage sag is stated as a percentage of the nominal voltage. Thus a voltage sag to 60% is equivalent to 60% of nominal voltage, or 264V for a nominal 440V system.
Sags are usually caused by system faults, energization of heavy loads or starting of large motors. A typical voltage sag is shown in figure below.
Figure 1.1 Waveform of Voltage sag
Voltage swell: A swell is defined as an increase to between 1.1 and 1.8 pu in RMS voltage or current at the power frequency for durations from 0.5 cycle to 1 min.
Voltage swell are caused by switching off a large load or energizing a large capacitor bank.
Figure 1.2 Voltage swell waveform
Impacts of Power Quality Problems on End Users
There are many ways in which the lack of quality power affects customers.
Short duration voltage variations have varied effects on consumers. Voltage sags (also known as dips) can cause loss of production in automated processes since a voltage sag can trip a motor or cause its controller to malfunction. For semiconductor manufacturing industries such a loss can be substantial. A voltage sag can also force a computer system or data processing system to crash. To prevent such a crash, an uninterruptible power supply (UPS) is often used, which, in turn, may generate harmonics. The protective circuit of an adjustable speed drive (ASD) can trip the system during a voltage swell.
Also voltage swells can put stress on computers and many home appliances, thereby shortening their lives. A temporary interruption lasting a few seconds can cause a loss of production, erasing of computer data etc. The cost of such an interruption during peak hours can be hundreds of thousands of dollars.
Harmonics, dc offset and notching cause waveform distortions. Harmonics can be integer multiples of fundamental frequency, fractions of the fundamental frequency (subharmonics) and at frequencies that are not integer multiples of the fundamental frequency (interharmonics). Unwanted harmonic currents flowing through the distribution network can causes needless losses. Harmonics also can cause malfunction of ripple control or traffic control systems, losses and heating in transformers, electromagnetic interference (EMI) and interference with the communication systems.
We can therefore conclude that the lack of standard quality power can cause loss of production, damage of equipment or appliances. It is therefore imperative that a high standard of power quality is maintained.
This project will demonstrate that the power electronic based voltage restoring device especially dynamic voltage restorer which is effectively utilized to improve the quality of power supplied to customers by mitigating voltage sag and swell in the Microgrid.
1.5 Literature Survey
R. Majumder, et al 1 proposed enhancement of power quality in a microgrid which may contain unbalanced and nonlinear loads, improvement of power sharing technique in microgrid with converter interfaced sources, stability analysis and enhancement in stability with supplementary controller.
Math H.J, et al 2 have discussed the power quality problems such as voltage sags and interruptions.
P. Boonchiam, et al 3 presents modeling and simulation of DVR including controls in MATLAB environment study and understanding such compensating devices. The DVR which is based on forced commuted voltage source converter (VSC) has been proved suitable for the task of compensating voltage sag/swells.
Ghosh, et al 4 this book describes enhancement of power quality using custom power devices.
R.C Dugan, et al 5 this book describes various power quality problems.
P. Kanjiya, et al 6 proposed a simple generalized control algorithm for the self-supported DVR is developed based on the basic SRFT. This novel algorithm makes use of the fundamental positive sequence phase voltages extracted by sensing only two unbalanced and/or distorted line voltages. The algorithm is general enough to handle linear as well as nonlinear loads.
Rosli Omar, et al 7 this paper discusses operating principles of dynamic voltage restorer and also the problem of voltage sag and swells and its severe impact on sensitive loads.
Yun Wei Li, et al 8 describes performance of dynamic voltage restorer and design of high performance control algorithms for DVR with improved robustness and desirable steady state and transient characteristics. In this paper, two voltage controllers are proposed for DVR.
Mohammad Faisal, et al 9 presents performance of dynamic voltage restorer is analyzed for two different controller such as PI controller and Park’s transformation based controller for mitigation of voltage sag.
Thomas Ackermann, et al 10 this paper gives a brief description on distributed energy resources and their advantages and necessities. And also discusses on impacts, location, capacity, issues of it in the system.
Shauna McCartney, et al 11 presents in detail on simulation and control of a wind energy system connected to the grid. Explains the controlling of wind systems and its impacts.
1.6 Objectives of the work
To model Microgrid as a combination of PV and Wind generation source with load change causing power quality problems.
To simulate Voltage sag and Voltage swell as power quality problems.
To Model DVR as mitigation device and investigate the effectiveness of DVR in compensating voltage sag and swell in Microgrid.
2.1 Solar Photovoltaic System
Working of PV Cell
The basic principle involved in working of a PV cell is the Photoelectric effect. . In this effect electron gets ejected from the conduction band as a result of the absorption of sunlight of a certain wavelength by the matter. In photovoltaic effect, when a photon particle hits a PV cell, some portion of the solar energy is absorbed in the semiconductor material as shown in below figure 2.1.
Figure 2.1 Working of solar cell
After receiving energy from sun, the electrons from valence band jumps to the conduction band, when absorbed energy is greater than the band gap energy of the semiconductor. Due to which electron-hole pairs are created. Now the electrons in conduction band become free to move. Movement of electrons create positive and negative terminal. These free electrons are enforced to move in a particular direction by the action of electric field present in the PV cells and also create potential difference across these two terminals. . These electrons flowing comprise current and can be drawn for external load. This current and the voltage produces required power when connected to external circuit.
Modeling of Photovoltaic cell
Single diode equivalent circuit model of a solar cell is shown in Figure 2.2 .I_L represents the light generated current in the cell, I_D represents diode current and I_Sh represents the current lost due to shunt resistance.
Figure 2.2 Equivalent circuit of solar cell
In Photovoltaic (PV) system, solar cell is the basic component. This photovoltaic system can generate direct current electricity, when is exposed to sunlight. Each Solar cell is similar to a diode with a p-n junction that directly converts light energy into electricity formed by semiconductor material. The output characteristic of PV module depends on the cell temperature, solar irradiation, and output voltage of the module.
Figure 2.3 I-V and P-V curve of solar cell
Figure 2.3 shows the output I-V curve and P-V curve of cell. It can be seen that a maximum power point exists on each output power characteristic curve. The equivalent circuit of a solar cell consists of current source, diode, a parallel resistor and a series resistor. The current equation of the solar cell is given as
I = Iph – I_d– Ish = Iph –Io exp (qV_d / nKT) – (V_d / R_SH)……………………… (1.1)
I_Ph = Photo current (A)
I_d= Diode current (A)
I_Sh = Shunt current (A)
V_d = Voltage across diode (Volt)
I_0 = Diode reverse saturation current (A)
q = Electron charge = 1.6X10-19 (C)
k = Boltzman constant = 1.38X10-23 (J/K)
T = Cell temperature (K)
Rs = series resistance (?)
Rsh = shunt resistance (?)
Power output of solar cell is P = V*I……………………………….. (1.2)
Wind Energy System
2.2.1 Wind Power Generation
Wind power is generated when the earth orbits the sun daily, it receives light and heat. Across the earth there are areas with different temperatures, so this heat transfers from one area to another area. Due to heat differences wind is created. In warmer regions of the earth, the air is hot and the pressure will be high, compared with the air in colder regions, where it is low. Wind is the movement of the air from high pressure to low pressure. From this the idea of generating power from wind came to action.
When compared with conventional sources, wind energy offers great potential to generate electricity without the causing any pollution problems. Mechanical energy in wind has been used for thousands of years for numerous applications like in milling, pumping water and other mechanical power applications. But the use of wind energy as an electrical supply with free pollution what makes it attractive and takes more interest and used on a significant scale.
Wind Energy Conversion System (WECS) includes either an induction type or a synchronous generator within its structure. The permanent magnet synchronous generator is considered as best choice for wind turbines due to the simplicity of its generating mechanism and good power density.
2.2.2 Components of Wind Energy Conversion System
Figure 2.4 Components of wind system
Various components connected to the wind systems are as shown in figure 2.4. Each of the elements is described below
Wind turbines produce electricity from kinetic energy of the wind to drive an electrical generator. Usually wind passes over the blades of turbine exerting a turning force over the blades. Inside the nacelle the rotating blades turn a shaft then goes into a gearbox. The gearbox helps in increasing the rotational speed for the operation of the generator and thus converts the rotational energy into electrical energy. Then the output electrical power is rectified by diode converters.
Power curve, which is a graph of power output versus wind speed is shown in figure 2.5
Figure 2.5 Power Speed Characteristics of Wind Turbine
Permanent Magnet Synchronous Generator (PMSG)
There are different types of generators are used for conversion of mechanical energy into electrical energy in wind system. But the most common is the permanent magnet generator.
Power Electronic Converter (AC-DC Recti?er )
It converts alternating current (AC) source to direct current (DC) source using power diodes or by controlling the ?ring angles of controllable switches. In this project, a three phase full wave AC- DC recti?er has been used connected to the output side of the wind model. It will convert the AC voltages generated by the wind model into DC voltage to make compatible with the DC side of the system.
Modeling of Wind generator
The output power of the wind turbine is given by
P_(0 =) 1/2?A V_wind^3 C_p(?,?) ………………………………… (1.3)
where Po is the mechanical power output of the turbine, C_pis the power coefficient of the turbine, ? is the tip speed ratio of the rotor blades, ? is the blade pitch angle, ? is the air density, A is the area of swept of turbine, V_Wind is the wind speed .
The power co-efficient of the turbine C_p (?, ?) is given by
C_p(?,?) = C_1C_2 1/? – C_3?? – C_4 ?^x – C_5 e^(?-C_6?^(1/?) ) ……………………………… (1.4)
Where the values of the coefficients C1 – C6 and subscript x depend on turbine type, ? is defined as the angle between the plane of rotation and the blade cross section chord, and ? is defined by
1/( ?) = 1/(?+0.08?) – 0.035/(1+?^3 ) ………………………………………………… (1.5)
2.3 Battery Energy Storage
Battery converts electrical energy into chemical energy for storing purpose. Batteries are charged and discharged. Basically it is a storage device which stores the excess power generated and uses it to supply the load in addition to the generators when power demand exists. Both solar PV and wind energy systems are integrated together i.e. connected to a common DC bus of constant voltage and the battery bank is also connected to this DC bus. Power transfer from the battery bank to the load takes place via this constant voltage DC bus. There are various types of batteries in the market like Lithium Ion, Lead-Acid, Nickel Cadmium, and Nickel Metal Hydride. High discharge rates are achieved by Lead-Acid batteries; these batteries offer a better solution for applications of energy storage. Nickel Cadmium (NiCd) batteries are better in all qualities and have low maintenance requirements than the Lead-Acid batteries.
2.3.1 Battery Terminology
Ampere-hour is represented in Ah. Storage capacity of the battery is measured by this unit.
E.g.: Suppose a battery delivers 10 Amp current for 10 hour is actually represented in 100 Ah.
It is represented as the ratio of nominal battery capacity to the discharge or charge time periods in hours.
E.g.: A 4-Amp charge for nominal 100-Ah battery would be represented as C/20 charge rate.
It is represented in KWh (Kilowatt-hour), which is actually a multiplication of rated capacity of battery in Ah with nominal battery voltage and further the combined value is divided by 1000.
E.g.: Suppose a Nominal battery voltage= 20 Electrical battery storage capacity= 50 Ah, then Energy storage capacity of battery (20V*50Ah)/1000 = 1.0 kWh
During Charge conditions battery receives current and during discharge conditions battery release current.
State of charge of battery is represented as SOC. This is very important parameter for battery, it de?nes as the amount of energy or charge in the battery represented in percentage of energy available in electrical fully charged battery.
SOC increases when battery is in charging process, and
SOC decreases when battery is in discharging process.
At any particular time, amount of capacity remaining in battery.
2.4 DC – DC Boost Converter
Boost converter or step – up converter increases input voltage magnitude to a required output voltage magnitude without the use of a transformer at the same time stepping down of current from the input side. There are various types of dc-dc converters that can be used to transform the level of the voltage as per the supply availability and load requirement. Some of them are:
1. Buck converter 2. Boost converter 3. Buck-Boost converter
Figure shows a boost converter consisting of an inductor, a diode and a high frequency switch. The switch can be typically a MOSFET, IGBT or BJT.
The control strategy lies in the manipulation of the duty cycle of the switch which causes the voltage control in the output. In this project boost converter is used for both solar photovoltaic and wind energy system to boost up the output voltage of the two renewables to the same level of DC bus voltage. Ripple in the input and output are reduced by filters made of capacitors or inductors at input and output of converter.
Figure 2.6 Schematic of a boost converter
2.4.1 Design parameters for boost converter
For a boost converter, the values of inductor, capacitor, load and the percentage of duty cycle is determined as follows:
In figure V_s = Supply voltage V_0= Output voltage
Duty cycle (D): It represents the fraction of the commutation period T during which the switch is on. It ranges between 0 (S is never on) and 1 (S is always on).It ca be calculated by knowing input and the required output for a boost converter.
D = 1 – V_s/V_0 * 100 ………………………………………. (2.4)
Inductor : The value inductance is determined as
L ? V_(s * D )/F_(s * ?I_L ) ………………………………………. (2.5)
? F?_s = Switching frequency in the range of 25 – 10 KHz