Abstract- Renewable Energy resources such as wind and hydro are regarded as one of the best solutions on extraordinarily increasing energy demand and that could be against recent climate change, which is mainly caused by those power plants burning fossil fuels such as coal, oil and gas. This resources used in Distributed Generation (DG), which is related with the use of small generating units installed at strategic points of the electric power system or locations of load centers, have attracted a lot of attention world-wide as same as renewable energy sources solely.
Hybrid system of wind and hydro generators could compensate each other if grid connected. Moreover, variable speed hydro generator could smooth the fluctuating power generated by wind turbine. Basically, nowadays there are two kinds of generators applying into hydro power plants: fixed-speed synchronous generator and squirrel cage induction generator (SCIG), and both of them could connect to an isolated load or grid without power electronic device known as converters. For wind energy conversion systems (WECS), permanent magnet synchronous generator (PMSG) and SCIG applied for electricity generation. Recently, the WECS have switched the wind turbine from fixed speed to variable speed which means the rotational speed of the machine is different from the wind speed. One of the obvious advantages of variable speed turbine over fixed speed one is the higher energy conversion efficiency, which is associated to the smaller in size, cheaper energy supplies and reduced visual impact for the same electricity generation. SCIG applied for wind generation part and synchronous generator applied for hydro power generation is discussed in this paper. The simplified structure of the hybrid system is showing and Simulations are done to verify considerations for this connection.
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Index Terms-Wind Energy, Micro Hydro, Distributed Generation, Inverter, Converter
DG resources are not limited just to one or two renewable energy source. Today, there are various different sources that can be used as a DG resource and each of them has a exceptional advantages and disadvantages. There is solar energy, fuel cell technology, and biomass and so on. But one of these sources is used more often and that's wind energy. This is mostly due to the fact that wind energy is a clean and widespread energy and its easier to use it than other resources in terms of efficiency and cost. But there is a certain disadvantage about using wind energy as a DG resource in power network and that is the problem of unpredictability of the wind speed. The wind energy operators cannot be sure that if the wind speed is enough to make a high rated electricity generation or its lower than what you have had expected. Due to this fact, there is a vital need for compensation of power and even voltage in networks connected to wind DG and it's often that wind and hydro turbines are connected to each other through a hybrid network to compensate each other. In hydro systems, a VSD generator is used which make the network able to smooth the fluctuations generated by a wind turbine. Basically, nowadays there are two kinds of generators applying into micro hydro power plants: fixed-speed synchronous generator and squirrel cage induction generator (SCIG), and both of them could connect to an isolated load or grid without power electronic device known as converters. For wind energy conversion systems (WECS), permanent magnet synchronous generator (PMSG) and SCIG applied for electricity generation. Recently, the WECS have switched the wind turbine from fixed speed to variable speed which means the rotational speed of the machine is different from the wind speed. One of the obvious advantages of variable speed turbine over fixed speed one is the higher energy conversion efficiency, which is associated to the smaller in size, cheaper energy supplies and reduced visual impact for the same electricity generation. SCIG applied for wind generation part and synchronous generator applied for hydro power generation is discussed in this paper.
In principle, wind energy is coming from the potential energy of the sun. The different temperature of ground in different area makes the wind moving. There are four obvious advantages of the wind energy: first of all, wind energy is clean. The air we breathing everyday are not harmful physically and no pollutions to the environment and human society. Secondly, the potential energy in wind form is very huge. The wind energy available around the world is talking about 2*107 MW, which is 10 times as the hydro energy. Thirdly, there is no shortage for wind energy. Lastly, there is no cost for generation. Nevertheless, the wind energy is not continuous and comes from every direction. Wind energy is varied in different season, different area, even different between day and night within one single day. These could be regard as a drawback of the wind.
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Generally, there are two ways to illustrate the power performance of the wind turbine in dimensionless form: fixed wind speed which is employing the power coefficient Cp and the tip speed ratio Î», and fixed rotor angular speed which is performed by the advance ratio J and the rotor speed power coefficient Kp. For the simplification purpose, the first way to perform the wind turbine is discussed in this paper. For a wind turbine of performance coefficient Cp, air density Ï, turbine swept area A and wind speed Vwind, the output power is:
To achieve the maximum value of the output power, the wind turbine needs to be operated under maximum power coefficient Cp, which is determined by tip speed ratio Î» and blade pitch angle Î². Maximum Cp is achieved when the blade pitch angle Î² is 0. The relationship between power coefficient Cp and tip speed ratio Î» when Î² equals to 0 is shown in Figure below.
Fig1. Rotor Power Coefficient Performance Cp against Tip Speed Ratio Î»
There are two converting steps in wind energy conversion system. The first one is to process the energy from wind energy to mechanical energy and the second one is from mechanical energy to the electric energy. Generators applied on the wind turbine are dealing with the second procedure. A lot of aspects should be considered in that step, such as the performance, efficiency, power quality and so on. Also, the second step could affecting the operation function and structure of the device in the first one. The squirrel cage type induction generators have been widely and increasingly used in wind energy systems and micro hydro energy systems for a few past decades . When an induction generator is operating, reactive power must be provided by a capacitor in parallel, SVC, STATCOM or synchronous condenser in the generation side, which could be viewed as a drawback determined by characteristics itself . In this paper, a synchronous condenser and a shunting capacitor are linked with the SCIG to keep the voltage level.
For an autonomous area, DC generators and some AC generators such as permanent magnet synchronous generator and Induction generator could be employed in wind conversion system to generate electricity. For a grid connected system, synchronous generator, Squirrel cage induction generator, doubly fed induction generator and so on could be applied into the wind generation system to supply the utilization. In this case, a Squirrel cage induction generator (SCIG) is selected and analyzed.
The SCIG is a traditional generator which is adopted for wind energy generation. When a SCIG in operation, reactive power need to be provided for the generator and keeping the voltage level. When an induction machine used as a generator, the rotor speed is a little bit higher than synchronous speed. Because the strength, rigidity and stability of the rotor are much higher, SCIG is suitable for wind generation. With the technology is more and more mature, SCIG is widely used in large wind farms and accounts for a huge percentage in wind generation. However, it could not utilize wind energy efficiently, so the conversion efficiency is lower than synchronous generators. But the performance of the induction generator is still improving. There is a high-performance induction generator wind turbine connected to utility grid is talking about in literature .
Induction machine is also called "asynchronous machine", comparing with any other generators, induction machine is simpler and easier to manufacture, utilize, and maintain, and it is more reliable and cheaper cost. However, the start and speed controlling is not as well as synchronous machines, and the power factor is lower, which could lay heavy upon the reactive power. Induction machine is the largest and widest machines applied around the world wide as a motor. Same as any other AC machines, induction machine have a stator and a rotor with a small gap between them.
The gap between stator and rotor plays a significant role in improving the performance of the induction machine. The gap of the moderate size or small size induction machine is around 0.2-2.0mm. The power factor will be improved by decreasing the gap, while it could be difficult to install the machine, unreliable operation and add some additional wear and tear. So the available minimum condition of the machine should be considered to minimum the excitation current.
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The excitation current will be provided by the capacitance device such as STATCOM, SVC or a Synchronous condenser, when the induction generator is connected to a load.
Grid Connection Power Electronic equipment
With the development of DG technology, the problems are solved by connecting DC-AC-DC converters. Because of the difference between the rotor speed and the magnet field rotation speed according to the load, the output frequency is different from the nominal frequency. An AC-DC-AC converter is necessarily applying on the wind energy system to reform the frequency so that could be connected to the grid as required. The AC-DC-AC converters are presented in the section behind.
When induction generator is connecting to a power system, a reactive power source is also needed to provide the reactive power to the generator in order to maintain the voltage level. Besides the generators, synchronous condensers, static condensers, static Var compensators, STATCOM could also be used as reactive power sources. Static condenser only can absorb the capacitive reactive power, while others could absorb both of the inductive and capacitive reactive power.
Synchronous condenser could be regarded as a synchronous motor with no load connection. When over-excitation occurring, it provides inductive reactive power as a reactive power source. When operating on under-excitation conditions, it absorbs inductive reactive power as a reactive load. Due to the practical needs and stability requirement, maximum under-excitation capacity has only 50%-65% of the over-excitation capacity. Synchronous condenser with automatic excitation system could change the reactive power absorption or output according to the voltage level of the installation point in order to adjust the voltage. When there is reinforced excitation equipment under a system break down condition, it could adjust the voltage as usual, and improve the system stability. However, synchronous condenser is a rotating machine with very complex operation and maintenance. The real power loss is very large and could be 1.5-5% of the full load condition. The smaller the synchronous condenser, the more expensive every 1KVA cost. So the synchronous condenser is always applied on bigger capacity condition. Moreover, the response of the speed is very long, and it is hard to fit the reactive control dynamically. Because of that, the synchronous condensers are replaced by SVC.
The reactive power of a Static condenser could be provided Qc is direct proportion to square of voltage level V. The equation could be demonstrated as follow in equation (2.5):
In this equation, Xc = 1/wc, which is the reactance of the static compensator.
When the voltage of the nodal point is getting lower, the reactive power to the system is getting smaller. When the system is under fault conditions or voltage drop owing to any other reasons, the voltage level will continue dropping. The adjusting reactive power function of the static condenser is not very good.
Static Var compensator (SVC) consists of a static condenser and a capacitor in shunting connection. The SVC has the ability to either draw capacitive or inductive current from the network . Capacitor could absorb reactive power and static condenser could output reactive power, the function of these two combine together could change the reactive power smoothly. When the voltage is changed, static Var compensator could adjust reactive power fast and smoothly to provide the need of the dynamic reactive power compensation. Comparing with the synchronous condenser, it is easy to operate and maintain, less in power losses, fast response, and strong in adaptability of inrush current. TCR and TSC can also compensate for single phase in three-phase system to adopt the unbalanced load varying.
STATCOM is more advanced reactive power compensator, which is applying a voltage source inverter with 6 GTO and reversed 6 diodes shunting together. Appropriate control of the GTO could transfer the DC voltage in the capacitor to the AC voltage to match the three-phase voltage in the power system. The AC part of the inverter is connecting the power system in parallel via a transformer or a reactor. Appropriate control of the output voltage of the inverter could change the STATCOM operation conditions in the situation of inductive load, capacitive load or no load.
A three-phase rectifier and a three-phase inverter are used in the AC-DC-AC converting system to reform the voltage magnitude and frequency. Power electronic devices such as rectifiers and inverters are employed into renewable energy systems to convert the power into the right forms for connecting to the grid, whose voltage and frequency are constant. AC-DC-AC converters are including a three phase rectifier, a three phase inverter, and a LC filter. In this case, a constant voltage output is given by uncontrolled rectifier, the LC filter in the DC link smoothes the DC voltage, and the voltage control is incorporate in the inverter. The DC bus voltage between two converters is maintained by the grid side inverter.
Switch mode inverter have been applied into many power system projects. It is claimed that uninterruptible power supplies, communication ring generators, aerospace power systems, and variable-speed ac machine drives use the switch mode inverter to change the dc voltage into ac voltage. The switch-mode is widely used into so many applications owing to the good behaviour during the steady-state period and dynamic period.
There are three main features of the switch mode inverters: firstly, in a VSI (Voltage source inverter), the dc side is a voltage source sometimes shunting with a large capacitor as a voltage source whose impedance can be neglected. But in a CSI (Current source inverter), the dc side connect a large inductance in series as a current source whose impedance can be neglected. Secondly, in a VSI, the output voltage in the ac side is a square wave owing to the clamping of the voltage source. The output of the current wave is various according to the different loads. However, in a CSI, the output current in the ac side is a square wave because the thyristor only change the current direction. At last, when the load is inductive, the reactive power needs to be provided. The capacitor or inductance at the dc side is used for reactive buffering effect.
The most widely used three-phase inverter is three-phase bridge-type inverter. Figure 3.9 give the information about the voltage source three-phase bridge-type switch mode inverter applied IGBT. This type of inverter can be treated as 3-phase half bridge inverter. As the same as the single-phase bridge-type inverter, three-phase bridge-type inverter is also turned on as appropriate for 180Â°in each device. The two branches at the same phase switch in turn, the fire angle of each phase is different from each other by 120Â°. So, there are three branches always working at the same time.
A PWM controlled three phase inverter is used as a grid side converter to invert the current in AC with frequency of 50Hz. The physical structure of the principle is showing below in Figure 3.9. With the pulse controlled open and shut of IGBT/diodes by PWM, voltage waveform is obtained after a LC filter.
For a grid connected inverter, the controlled pulse signal for the inverter could be built by an output controller, a PWM Generator and a Unit Delay to control the AC output voltage stability. The physical structure of the output controller is demonstrated by Matlab software is visible in Figure below.
Fig. 3 Subsystem of the Output Controller for AC Voltage Side Controlling of the Inverter
Hydro energy is regarded as a renewable energy owing to the non exhausted existing potential energy of the water. The difference between hydro energy and fossil fuel and nuclear plants is that there is no second conversion from heat to electric power.
There are a lot of advantages of hydro energy over power plants using fossil fuels and nuclear fuels. First of all, owing to the periodicity of the hydro energy, it could be obtained continuously. Fuels used in coal fired plants and nuclear plants are non-renewable. Utilization of hydro energy efficiently could reduce the consumption of the fossil fuels, increasing the limited time of oil and coal exploitation and treat the environment friendly. Secondly, hydro energy could be used comprehensively. Most of the hydro generation plants could achieve preventing or controlling floods, irrigation, shipping, water supplying, fishing and so on besides electricity generation. The water flows in the upstream could also provide water supply and energy to downstream hydro power plant. Thirdly, the cost of the energy supplied by hydro plant is lower and hydro energy is more efficient than fossil fuel power plant. No transporting and exploitation cost in hydro energy and no fuels are need in hydro generation. Also, equipments applied on the hydro turbines and accessories are not large in quantity, so fewer people in maintenance and operation, which could reduce the capital cost and operation cost. Fourthly, hydro energy has much higher efficiency than that of fossil fuels. Generally speaking, the efficiency of the hydro conversion could have 85%, with a small percentage of losses. Hydro energy is regarded as the least cost energy for the electricity generating. Fifthly, comparing with any other forms of the energy conversion technologies, there is no pernicious gases, dust, and any other polluting emissions comes out, less waste water, no chemical contamination and heat waste, no danger of the nuclear radiation. It could improve the environment and be beneficial to the existing of human beings. Last but not least, comparing with the electric power, hydro power could be stored via the reservoir, while the electric power could be not stored. When the rainfall is coming, the coming water is much more than usage and the rest of the water is stored in the reservoir. If the water utilization for generating electricity is less than the water supply, the reservoir could be used as backup to generate electricity. This could adjust the balance between provide and demand. And it also could improve the flexibility and economy of electricity generation.
Grid Connection of Hydro turbine
Generally, hydro energy based on the potential energy in water fall could provide the hydraulic turbine a constant value of energy no matter how the external factor change such as temperature decreasing. The prime mover of hydraulic turbine can be used to drive synchronous machine or permanent magnetic synchronous machine. In this case, synchronous machine is selected as a generator applied in the hydro energy system.
The rotor and magnet are rotating at the same speed in a synchronous generator. The relationship among the rotor speed n, frequency f and the pole pair number p can be illustrated in equation (4.1).
For a synchronous generator connected to hydro turbine, the rotation speed cannot be much higher as generators employed in the fuel fired plant, so the pole pairs should be set higher. In this case, the number of pole pairs is set to 20. According to the 50Hz system, rotor speed is 150 rotations per minute. The phasor diagram of the synchronous generator connected to the grid is shown as follow in Figure 4.2:
WhereÎ´1 is the angle between the grid voltage U and the generator electromotive force E0 with no load, and Î¸1 is the angle between grid voltage U and grid current I.
Grid connection could be viewed as many synchronous generators operating in parallel, which have many advantages to provide energy to the load. At the first beginning, the parallel operation function could improve the power quality of the utilization. at the present, the capacity if the grid is big enough to bear the disturbance and turbulence of the power system owing to reasons such as fault occurring, losing some generator unit in the power system, or problems in the transmission line. Just one single generator's capacity is much smaller than that of grid. And the generators connected to the grid in parallel make the voltage and frequency as a constant. If a synchronous machine is connecting to the grid, there is definitely no effort for the whole systems voltage and frequency, which could be presented by mathematics equation 4.2 as:
In the UK, the frequency is 50Hz. The grid with constant frequency and voltage could be viewed as an infinite grid for a single generator. Secondly, the electric power could compensate each other and reduce the stored capacity of the plant for backup. Thirdly, reduce the unnecessary generators connecting to the grid according to the regulation of the varying load in different season even different utilization in one single day, and reduce the cost. Fourthly, the power plants could improve their interests according to the increasing demand. Last but not least, the reliability of the power supply will be improved. One single generator operation function and one large capacity demand is not affecting the grid too much owing to the big capacity of the grid.
When a synchronous machine is connecting to the grid, inrush current will not occur in the circuit after a short time any more. To achieve this, the gird connected generator should be suitable for 4 conditions, which is illustrated as follow:
The frequency of the generator should be equal to the gird frequency.
The amplitude of the voltage and the gird side voltage should be equal to each other.
Phase sequence of the generator and the phase sequence of the grid should be equal to each other.
The phase angle of the voltage and phase angle of the grid should keep the same.
Synchronous machine could be classified as two categories, one is the non-salient pole generator, and the other is salient pole generator. The air gap of the non-salient pole generator is uniform, while that of the salient pole generator is not. For a high speed synchronous generator, which means the speed of rotation is more than 3000r/min, non-salient pole generator is applied to generate the electricity. However, the rotation speed could not reach such high, salient pole generators is applied. For the hydro turbines, salient pole generators are applied in the conversion system owing to the hydro turbine is a low speed prime mover.
The aim of the generator connecting to the grid in parallel is to provide power to the grid. Generally, after the generator connecting to grid for a short while, the generator still could be viewed as the same as no load connection. At that moment, the input mechanical power of the generator is equal to the no load loss, without any part transforming to electromagnetic power. If the input mechanical power is increasing, which means the mechanical power is greater than no load loss, the rest of the power could be transformed to electromechanical power, and the generator will provide active power to the grid. So the active power out from the generator is transformed by input mechanical power. The input mechanical power should be changed relatively to change the active power output from the generator. Thus it can be seen that if the active power of the synchronous machine connecting to the grid in parallel needs to be adjusted, just change the input mechanical power. At the same time, the voltage angle Î´ will change automatically, and the electromagnetic power and output power will change relatively, then a new balance is established. The steady-state characteristics are showing in Figure 4.3. And the process is demonstrated below.
Direct current via the excitation windings must be provided by excitation system when synchronous machine is in operation. The excitation current is often supplied by a small dc generator called an exciter . Excitation system could keep the output voltage level of the generator and gird voltage level according to the load. Excitation system is one of the important parts of the synchronous generator. The excitation system needs to provide functions as follow:
Excitation current needs to be provided to the generator in normal operation, and could adjust the excitation current relatively to maintain the terminal voltage of the generator or the grid according to the load performance.
When the voltage of the system is drop extraordinarily owing of the short circuit or any other reasons, excitation system could exciting the generator to improve the stability of the power system.
Excitation system could reduce exciting current to limit the terminal voltage of the generator when losing some loads.
Excitation system could extinguish or reduce exciting current to reduce the damage level of the facilities when short circuit occurring inside the generator.
Excitation system could adjust the reactive power and distribute reactive power when there are two or more generators connecting to the grid.
The reaction speed of the excitation system needs to be very fast and reliable in operation, simple in structure of the equipments, easy to maintain and fix and so on.
A 25kV grid connected wind and hydro hybrid system with a synchronous generator connected with hydraulic turbine and a SCIG connected with wind turbine is demonstrated below.
A synchronous condenser and a shunt capacitor are connected with wind generation system based on a SCIG to provide active power. LC filter associated between the AC-DC-AC converters to filter the DC ripple. The main function of Voltage regulator is to providing a stationary AC voltage as an output controller, which is linked together with a PWM generator and Unit delay to produce the controlling pulse. A three-phase inductance and capacitance together as an AC filter are applying to isolate and filter the harmonics. A step up transformer levels up the voltage from 500V to 25kV to feed the grid. Simulation results are shown below. The main simulation circuit is shown on the next page.
Nowadays, the capacity of the grid is very huge, moreover, the voltage and frequency is fixed no matter how big is the varied load or the disturbance owing to reasons such as fault occurring, losing some generator unit in the power system, or problems in the transmission line.
For the hybrid system associated with DG technology, grid connection could support the load reliably. In addition, for economical consideration, the surplus energy could be sold back to the grid. A 25KV 50Hz power system is created to present the grid.
A fixed time Ts = 2*10-6s discrete step is selected and ode45 (Dormand-Prince) in solver to simulate the model.
In this paper, the performance of wind and hydro hybrid system connected to the grid is discussed and simulated based on matlab. Synchronous machine, induction machine are used to present the wind and hydro energy generation system from matlab. Rectifier and inverter are modelled with matlab individually.
In this Hybrid system of wind and Hydro, small capacity of the wind the hydro turbines and generators is talked to build a micro generation for grid connection. The simulation results show that it is possible to build such kind of generation facilities. However, it could be limited by harsh environment and cost. With the development of the wind and hydro hybrid technologies, hopefully the price will drop. Individuals could afford the facilities for micro generation within their homes. All types of equipments could be selected for options to improving energy efficiency.
Because of large elements from sim power system and simulink in matlab, some bugs will occur even if just one setting is changed. For example, when the system losing the grid support, the synchronous generator behaves like lacking of grounding system; the ratio of the three-phase transformer does not match with the simulation result, different performances with same setting in different type, such as three-phase transformer 12 terminals element and three-phase transformer (two windings) element. Future works could be done for the model to consummate the project. Owning to the complex settings the model will be run very slowly during continuous condition. Here only a discrete model is built, to get more accurate result, larger or more advanced computer is necessary.
With lacking of the fossil fuels and raising awareness of the climate impact, more and more renewable resources could be applied into the generation system and hybrid system will play a significant role to deal with energy conversion in the near future.