Integration of renewable energy in electric power system

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The increasing importance of fuel and rapid growth in the demand has been responsible for the revival of keen interest in alternative sources of energy. These green sources or commonly known as renewable energy have caught the eyes as a modern, environmentally friendly and more efficient to the traditionally used fossil fuels.

With the growing interest in the renewable system especially in the wind turbines and micro-hydro generators as an efficient, green, clean energy source, the induction generators are being considered as a better and alternative choice to the well-developed synchronous generators because of their low cost construction, maintenance, inherent ruggedness and operational simplicity. The induction generators are capable to generate power at variable speed making its use in high class application in various modes such as self-excited stand-alone system. A number of renewable energy sources like wind power, solar power, hydro power, industrial waste, geothermal, biomass are the mainstream of the renewable system. Since small hydro and wind energy sources are available in excess amount, their utilization was felt quite promising to accomplish the future energy requirements.

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The project work has been devoted to the study of the renewable power system mainly involved with the induction machines. The induction motor, workhorse of the industry has been studied, investigated and used to generate the electric power. A detailed study of the performance of the induction generator operating in the above referred modes during steady-state and various transient conditions is included in the project study.

1.2 INTRODUCTION:

First of all, “Integration of green or renewable energy in electric power system” is one of the most interesting, fascinating, attractive and bewitching concept for a final year project. This project has been an active subject for the course mates and other contenders for final year project have been overshadowed.

1.2.1 What is renewable or green energy?

Renewable energy is effectively uses natural resources such as wind, sun, tidal, rain, and geothermal heat which are naturally replenished and comes from earth. The provision of energy through natural resources is modern and cleaner which contributes to the national development and reduces the negative impacts which are destroying our planet. The modern renewable technologies convert these resources into productive form of energy such as electricity, heat, chemical or mechanical power.

1.2.1 Why renewable energy is important?

The fossil fuels such as coal, oil and natural gas are playing a vital role for meeting the energy needs. No doubt that it is quick and easy to use them but these energy sources are being used more rapidly than they are being created. They will be run out in the near future and also the use these resources are putting our planet into high risk by means of global warming and waste disposal problems. Using the renewable energy can minimise the negative effects on globe and can fill the gap between the demand and supply of the energy.

The mainstream forms of the renewable energy are wind power, solar energy, hydropower, biomass, bio fuel, tidal energy and geothermal energy.

The production and distribution of energy using the modern energy technologies have a central role in the social and economic development at all scales, from household and community to regional, national and international. Apart from its welfare effects, energy is closely articulated to environmental pollution, degradation and quality of life. Today, we are most majorly dependent on non-renewable fossil fuels that have been and will pursue to be a major cause of pollution climate change or simply known as global warming. Our dwindling supply of fossil fuels or petroleum and the horrible effect of their usage on environment are alarming for upcoming generations and because of these problems, discovering sustainable alternatives is becoming crucial. Moreover, the biggest challenge in realising a sustainable future is to develop technology for integration and control of renewable energy sources in smart grid distributed system.

1.3 PROJECT AIMS AND OBJECTIVES:

1.3.1 AIM:

The basic aim of this project is to run an induction machine as a generator to generate the electric power and to understand the functioning of rectifier by means of simulation on PSPICE.

1.3.2 OBJECTIVES:

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The objectives of this project is to carry out a detailed investigation of integration of renewable energy into electric power system in the range of 100's of watts to kW's. The project mainly involves in;

  • The detailed study of renewable power systems
  • Overview of wind power system
  • The study of AC and DC systems following by;
  • Designing, testing and simulation of a AC to DC rectifier (e.g. Matlab, PSpice)
  • Building and operating the induction generator by using an AC motor
  • Suggesting the possible ways of connection to electric power system i.e. Grid connected system
  • Carry out a critical review of the ways to maximise the efficiency of the system and suggesting the ways of energy storage

Renewable energy systems are modern, exciting, fascinating and remarkable subject. This is one of the interesting sectors of modern science and engineering where governments are investing a lot to discover the alternative sustainable energy sources. Therefore, the main goal of this project is to carry out a detailed research of integration of renewable systems or green energy into electric power system in order to understand the effectiveness of this idea.

1.4 SUMMARY:

The chapter covered the main points of the study and gives reader a better understanding of the project by depicting the aims and objectives of the work, which will be covered later in this report.

CHAPTER 2

Overview Of The Renewable Systems

2.1 HISTORY OF RENEABLE POWER SYSTEMS:

Energy has always occurs in one form or another and it can be transformed from one form to another but cannot be destroyed or created. For example, car uses stored chemical energy into mechanical energy to move the car. The stored chemical energy in a flashlight battery becomes the light energy when the flashlight is turned on. The green or renewable energy cannot be used up and also they have a very little effect on the environment which means its use cannot be restricted. A renewable energy source is replenished continuously but on the other hand, the fossil fuels like uranium, petrol or coal will be run out in the near future.

There is no shortage of the renewable sun because they are mostly taken from the sun, wind, water, tides, plants, and biomass and used to produce electricity and fuels. Research and studies has that the energy produced from the wind power is more than twice as much as demand will be in 2020. The fast deployment of renewable energy technologies in near future will raise the challenges and their opportunities regarding about the integration of green energy to the electric system.

Until 1980s, the interest in the green energy was limited primarily among the private investors. However, concern of environmental issues, the consideration of fuel diversity and dubiety of traditional fossil fuels are becoming important factors into today's electric utilities system and resource planning, renewable energy technologies are catching the eyes to find their position in utility resource portfolio.

The worldwide demand of electricity is projected to increase from 12 trillion to 19 trillion kWh in 2015 which constitutes the world average growth of 2.6 %. On the other hand, the growth rate in developing countries id estimated to be 5%. Today, most of the energy demand in the world is fulfilled by fossil fuels and nuclear power plants. Only a small percentage of the energy is obtained by green energy technologies, such as wind, solar, biomass, geothermal and ocean. Among the renewable power sources, wind and solar energy have experienced the distinguished rapid growth in the past decade. One of the welcoming factors for the renewable energy (especially wind energy) is the decrement in the cost of renewable power system which has declined by 80% since the early 1980s, which has resulted in the cheap option as compared to the fossil fuels. Local renewable power plants near the consumption point can also benefit in reducing the voltage drop at the end of the long overloaded line. The on-going research on green energy is aimed to bring the cost relatively low by next decade which is highly competitive with the energy cost of conventional power technologies. For these reasons, wind power plants are distributing economical clean power and expected to find importance in the energy planning in many parts of the world.

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While planning for renewable systems and integrated energy development in the future, environment itself must be considered, also the factors like the existence of energy resources, system needs, local needs where it is desirable to install a renewable energy resource should be monitored carefully. The capability of the grid supply, the electrical and mechanical behaviour of the load and economical and environmental effects on the region describe that how successful the investment may be.

2.2 WIND POWER GENERATION SYTEM:

Let's take an example of a renewable system and here we assume the brief explanation of wind turbine system. The following diagram reveals the complete process and steps involved to generate electric power from wind.

Here is the simple example of how does a renewable power system work. The system involves many steps which are;

  • Wind turbine runs at variable speed
  • Variable current in the stator of the generator
  • Conversion of variable frequency AC to DC by using thyristors or large power transistors
  • Inverter is used to convert the DC to AC at fixed frequency
  • Filtering the AC waveform.
  • Stepped up by the transformer

The blade of the turbine are adjusted in the way that they faces the wind, when the wind blows in , it causes blades to move which rotates the shaft connected to it as well in a nacelle. The shaft goes into the gearbox which increases the rotation of the generator. The generator uses electric and magnetic fields to convert the rotational energy to electric energy. This electric energy is then sent to the step up transformer where the voltage is stepped up to thousands volts to the substation.

2.3 SUMMARY:

To conclude, fossil fuels are running out rapidly with the increase demand of power and now are the time to invest to find the alternatives for energy resources in order to balance the supply and demand. And renewable energy resources have been a better choice to the petroleum and fossil fuels because of their efficiency and environment friendly characteristics.

CHAPTER 3

Experimental Work

THE FULL-WAVE BRIDGE RECTIFIER (AC TO DC CONVERTER):

3.1. INTRODUCTION:

In this chapter, I will focus on the importance of the rectifier and inverter topology for distributed generation (DG) as an approach in integration of green and renewable energy sources. Furthermore, I will be discussing the characteristics of a full wave bridge rectifier by means of designing, simulating and operating using a PSPICE computer based software. Finally, I will include the calculations of important factors which are required to achieve to have a better understanding of the power system.

3.2 DISTRIBUTED GENERATION SYSTEM

The demand of electric power is never ending. Along with the rapid growth in electric power demand, sustainable development, climate and environmental issues, and power quality and reliability have become concerns. The titled system uses many small generators of 2-50 MW output power, normally situated near the consumer locations by reducing the voltage drop. Environmental friendly renewable resources such as PV cells, clean and efficient wind electric generators and hydrogen electric devices (fuel cells) have given great opportunities for the development of distributed generation.

The above discussed DG system requires power electronics interfacing and different methods of control and conversion of operating system such as voltage from one type to other. When connected to grid system, the DG unit should be able to provide steady, low regulation error, low total harmonic distortion (THD), and fast response AC power under various load requirements.

A DC/AC voltage source inverter is most commonly used electric device for a renewable system, which involves many technologies and control aspects under different operating conditions. The satisfactory results are only achieved when two or more inverters are conducting, which also involves active P and reactive Q power control under different load characteristics and operating conditions.

3.3 RECTIFIER CONTROL IN CONTROLLED AC-DC-AC SYSTEM:

3.3.1INTRODUCTION AND IMPORTANCE:

In a distribution generation environment from a renewable/green power system, the significant portion of feeders and voltages are controlled by AC/DC rectifiers. The supply source can be a wind turbine or a gas turbine driven generators or other combinations of AC and/or DC systems. From a source only 3-phase balanced current is required which is normally know as power factor correction (PFC). For a PFC, there are many rectifier technologies which can be used to convert and conduct the required voltage.

For high power applications, such as wind turbine systems where especially high performance is required, a continuous conducting mode (CCM) boost rectifier also known as PWM rectifier is introduced in the system for its high efficiency, good quality current and low EMI emissions. A standard continuous conducting mode rectifier has similar characteristics to a three phase full bridge rectifier and uses the full bridge topology. A high control performance is achieved when a DC voltage is boosted greater than the input line voltage amplitude. The performance can be maximised by introducing a split DC bus technology with a three level controlled rectifier, which regulates both the top and the bottom half voltages of a DC bus. The full bridge diode rectifier can be implemented in with a modified voltage regulation scheme based on the standard approach.

In a three phase system, when the full wave bridge rectifier is used with an inverter, it is essential that the effect of the inverter on the system is monitored carefully for an excellent result. One of the frequently impact is unbalanced load on the inverter side; the DC voltage ripple problem is either caused by the unbalanced load current or unbalanced input voltage supply. It has been seen that once the inverter load is unbalanced, neither the inverter input power nor the inverter output power remains constant. The existence of harmonics in DC bus voltage can be shown in power converters by using a switching function.

Power control in a distributed generation (DG) is one of the key factors when the grid-connected method is used. Use of a controlled rectifier should not only provide well-regulated DC voltage but also function as power factor correction and take balanced input current. In a three-phase system, unbalanced inverter load may introduce ripple DC voltage which causes the unbalanced input current for the rectifier.

3.3.2 WORKING PRINCIPLE:

A rectifier circuit converts AC voltages into the unidirectional DC voltages. When a rectifier is equipped with a filter to help smooth the output current, this type of combination of rectifier and filter is often referred to dc power supply. If a rectifier is fed from a single phase supply, it is single phase converter, otherwise the rectifier connected to the multiphase i.e. three phase supply is known as three phase rectifier. If the converter employs only diodes which are non-controlled switching devices, it is called a non-controlled rectifier. By employing the controlled switches i.e. thyristors, it is a controlled rectifier. Converters that consist of both controlled and non-controlled devices are known as half-controlled converters. In addition, the AC-DC converter can be classified according to the number of pulses of current passing through the load during one cycle of the source voltage.

3.4 SIMULATION OF RECTIFIER CIRCUIT:

3.4.1AIM:

The aim of this chapter is to Simulate and investigate the characteristics and operation of a full-wave diode bridge rectifier by using ORCAD / PSPICE.

3.4.2OBJECTIVES:

The main objectives of the lab are to analyse, simulate and discuss the operation of a two-pulse non-controlled bridge rectifier.

3.4.3 THEORY OF OPERATION:

The Full wave rectifier PSPICE circuit consist of four diodes which are connected in the form of electronic bridge. For a sinusoidal input voltage, when the input is positive, the diode conducts and the output voltage is same as the input; i.e. the diode is functioning as the forward biased it is in the reverse direction it will not conduct.
The capacitor here used is the electrolytic capacitor. It is used to filter the ripples, which are present in the output of the diode bridge. The resistor is connected in the circuit form which the output is obtained. The diode here used is 1N4500.

3.5 CALCULATION:

[1] DC value of the output voltage waveform:

The dc voltage of the output waveform is given by the formula;

Vdc = 2Vmπ ,

where Vm is the maximum value of the output voltage

Vm = 2 (Vrms)

=2 (40)

= 56.57 volts

Substituting values in Vdc = 2Vmπ gives;

Vdc = 2 (56.57) / π

= 36.04 volts

[2] DC value of the output current waveform:

The average or mean current of the output current is given by;

Idc= 2VmπR

= 56.57/ 50(3.1415)

= 0.72 amp

[3] DC output power:

Pdc = IdcVdc

= (36.04)(0.72) = 25.94 watts

[4] The RMS output voltage:

The rms value of the output voltage is given by

Vrms = Vm2

=56.52/2 = 40 volts

[5]The RMS output current:

The rms value of the output current is;

Irms = Vrms /R

= 40/ 50 Ω = 0.80 amp

[6] The AC output power:

Pac = Vrms Irms

= (40)(0.8) = 32 watts

[7] The Efficiency of the rectifier:

The efficiency or the rectification ratio is the ratio of DC power to AC power.

Efficiency = (DC output power / AC output power) x 100%

=Pdc Pac x 100%

= (25.94 / 32) x 100% = 81 %

[8] Form factor:

The Form Factor is the ratio of Vrms to Vdc and it is the quantity of the measure of the output voltage waveform.

F.F. = Vrms / Vdc

= 32 / 36.04 = 1.11

[9] Ripple Factor:

It is the measure of the ripple content of the output voltage waveform and is given by the function

R.F. = Vac Vdc

= (V ²rms - V²dc) / Vdc or

=FF²-1) where FF= 1.11

= 0.48

[10] RMS supply current:

Irms = Vrms / R

= 32 / 50

= 0.80 amp

3.6 SIMULATION:

3.6.1 RESULTS:

Figure 4 Simulation

The current through the load is unidirectional and to explain this, During the positive half cycle, the diode D1 and D4 behave as forward biased and conducts the flow of current and when the supply is negative, i.e. during the negative cycle of the wave the other two diodes function as forward biased and conducts.

So in this manner, the diode keeps switching the connections and so current only flow in one direction through the resistor or load.

So the full wave rectifier bridge circuit is more common approach to manipulate the AC input voltage in which both halves of the cycle are used to give an output current which is unidirectional.

3.6.2 CAPACITANCE EFFECT:

CAPACITANCE: (500uf):

The variation or the fluctuation in the output voltage can be reduced by employing a capacitor in parallel to the load of the output voltage and the function of the capacitor is to filter the ripple voltage.

Figure 5 Capacitance effect on rectifier

Capacitance Effect - Figure 6 Rectifier Simulation with 500uF capacitance

Figure 7 Rectifier Simulation showing first five Harmonics with 500uF capacitance

The first five harmonics of the output voltage are measured and also peak to peak ripple voltage is measured and calculated.

Frequency (Hz)

100

200

300

400

500

Output Voltage

7.9481

1.8975

825.013m

1.0812

740.220m

Measured ripple voltage = 54.053 - 41.074 = 12.979 volts

Calculated ripple Voltage = Vripple = Idc2fC where Idc 0.72 amps

= 14.4 volts

CAPACITANCE: (100uf):

Capacitance Effect-Figure 8 Rectifier Simulation with 1000uF capacitance

Frequency (Hz)

100

200

300

400

500

Output Voltage

5.940

1.340

1.339

1.067

500.46m

Measured ripple voltage = 53.894 - 46.615 = 7.278 volts

Calculated ripple Voltage = Vripple = Idc2fC where Idc is 0.72 amps

= 7.20 volts

CAPACITANCE (2000F):

Capacitance Effect- Figure 10 Rectifier Simulation with 200uF capacitance

Figure 11 Rectifier Simulation showing first five Harmonics with 2000uF capacitance

Frequency (Hz)

100

200

300

400

500

Output Voltage

4.8997

1.4685

1.4733

875.99m

729.16m

Measured ripple voltage = 53.913 - 49.947 = 3.966 volts

Calculated ripple Voltage = Vripple = Idc2fC where Idc is 0.72 amps

= 3.6 volts

3.6.3 DISCUSSION:

The variation or the fluctuation in the output voltage can be reduced by employing a capacitor in parallel to the load of the output voltage and the function of the capacitor is to filter the ripple voltage.

In this circuit the capacitor is used as the bank of charge. During the second half of the cycle, As the capacitor is discharging supplying the load current, its voltage is dropping down, and when the supply voltage is greater than the output voltage, the diode are forward biased again and conducts current. This means the cycle repeats and hence the output involves the ripple voltage which is peak to peak to voltage. The best output voltage is one which has the low value of ripple voltage or ideally we wantVripple=0.

Capacitance

Measured Capacitance

Calculated Capacitance

500 uF

12.979 volts

14.4 volts

1000 uF

7.278 volts

7.20 volts

2000 uF

3.996 volts

3.6 volts

It is clearly shown in the above plots and table that the bigger or larger the value of capacitance, the more charge can be stored and the lower the low ripple voltage. Furthermore, large value of capacitance to minimise the ripple voltage will also result in reduction of the conduction interval of the diode. The time it takes to discharge the capacitor depends on the time constant T = RC.

3.7 LINE CURRENT VS RESISTANCE:

FIRST FIVE HARMONICS OF THE LINE CURRENT FORDIFFERENT VALUES OF LOAD RESISTANCE AT A SMOOTHING CAPACITANCE OF 1000 uF:

3.7.1 RESULTS:

When load resistance is 30 Ω

Frequency (Hz) Harmonics

50

150

250

350

450

Line Current (A)

2.3097

1.705

1.2569

780.20m

504.571m

When load resistance is 50 Ω

Frequency (Hz) Harmonics

50

150

250

350

450

Line Current (A)

1.2837

1.1573

937.163m

680.543m

455.489m

When load resistance is 100 Ω

Frequency (Hz) harmonics

50

150

250

350

450

Line Current (A)

668.315m

629.103m

555.400m

459.861m

357.320m

3.7.2CONCLUSION:

Resistance

Measured fundamental current harmonic

30 ohms

2.0937 amps

50 ohms

1.2387 amps

100 ohms

668.315 m amps

The above plot and graph show that the larger the value of the resistor or load the less is current flows through the resistor which is drawn through the capacitor. So, for the smaller ripple voltage, the time constant must be large as compared to the time period of the input voltage.

3.8 DISCUSSION:

This AC to DC conversion process usually involves a step-down transformer, rectifier, and a filter. A filtering process is normally improved by using a capacitor having a smooth capacitance (1000uF in this case).The step down transformer is used to supply the line AC voltage and to decrease it to the required low value of DC voltage. The output voltage from the step down transformer is then supplied to the rectifier which rectifies only the positive halves of the input. The purpose of the filter is to smooth the rectifier output to obtain the constant DC voltage output.

A regulator can be used to improve the load regulation of the rectifier to obtain a constant output voltage in spite of the changes in input voltages and load current. This can be done either using a Zenor diode circuit or a voltage regulator, Zenor diode circuit functions as the voltage regulator when the diode is operated in breakdown region and it employs as reverse biased. To keep using Zenor diode as voltage regulator it must be operated in a breakdown region at a current greater than knee current. The knee current is the reverse current at which Zenor diode enters to the breakdown region, Zenor maximum current is the maximum current at what the diode can function without any damage. So the good regulation is obtained when the diode function as reverse biased between the knee current and knee maximum current. The second way of improved the regulation is the use of a normal voltage regulators which are very efficient.

3.9 SUMMARY:

To summarise this chapter, we discussed about the great importance of the controlled AC-DC-AC conversion which involves the functioning of the rectifier. The role of rectifier along with the filter has been discussed and the parameters on which the operation of rectifier depends such as resistance and capacitance were explained in detail.

CHAPTER 4

Induction Motor As Induction Generator

4.1 INTRODUCTION:

Just like DC machines and synchronous machines, the induction machine can be used as generator or as a motor. Because of the poor performance of the induction generators, they were not commonly used in the industry. In recent years, however, the induction motors and generators have been used for variety of purposes for their beneficial use and now they are known as workhorse of the power industry. This chapter will describe the principles of operation, investigation and performance analysis of an AC induction motor.

4.2 INDUCTION MACHINES:

Induction machines are used mostly as motor. They are seldom used as generators because of their poor performance characteristics and unsatisfactory results as generators for most applications. As motors, on the other hand, they are called workhorse of the industry. The primary advantage of the induction machine is brushless construction and no need of separate DC power supply. The disadvantage of both synchronous and DC machines are omitted in the induction machine, resulting in low cost construction, maintenance and better transient performance.

The induction machines need an AC excitation current. These machines are either self-excited or externally excited. This excitation is normally done by the use of capacitors or starting resistors. Many wind power system use induction machines as the electrical generator for their economy and reliability. These are often called the rotating transformers and either can be one phase or three phase induction motors.

THEORY OF ELECTROMECHANICAL CONVERSION:

A transformer may be explained as an energy transfer device, where energy is transferred from the primary to the secondary without changing its form of energy. The electrical energy is carried in both sides of the transformer. On the other hand, DC machines, synchronous machines and induction machines are devices used to convert energy mainly from electrical to mechanical and vice versa. They convert either mechanical energy to electrical energy known as generators, or electrical energy to mechanical energy in the case of motors.

4.4 CONSTRUCTION AND WORKING PRINCIPLE:

An induction consists of a rotor and a stator. The stator is piece of laminated core similar in transformer, with conductors embedded in slots. These conductors when energised with AC power supply provide a varying magnetic flux density. The rotor is cylindrical, mounted on bearing and separated by the stator by a small air gap.

Voltage is induced in the rotor conductors and this induced voltages cause current to flow. Interaction of the stator and rotor magnetic fields produces an electromagnetic torque. This torque causes mechanical load to rotate.

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The angular speed of the rotating magnetic field is called the synchronous speed and given as

Ns = 120 x fp

Where f = frequency and p = number of poles

The stator magnetic field is rotating at the synchronous speed and as a result it is represented by the rotating magnets. The effect of the relative speed between the stator and the rotor induced the voltage with the change in the flux density. The magnitude of the induced voltage is given by the Faraday's law of electromagnetic induction, which is;

E=-dø/dt

4.5 OVERVIEW OF INDUCTION MOTOR:

The induction motor has always been a machine of interest regarding its control in both the power electronics and control community. Induction motor is an induction machine which is powered directly (to the stator armature windings) by the alternating current AC and (to the rotor windings) by induction or transformer action from the stator. These motors follow the same function of a transformer where voltage is induced in secondary windings because of the effect of primary windings. Hence, these are also known as rotating transformer. The stator windings are similar to the stator windings of the synchronous machine. However, the rotor of the induction motor may be either of two types;

  • Wound rotor
  • Squirrel cage-rotor

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AC Motors can be classified into two categories i.e. Single Phase AC motor and Three Phase AC Motors. These types of motors have no brushes unlike DC motors and work only on alternating current (AC). They may range in size from 1/4 horsepower up to 5 horsepower or more.

4.6 SINGLE PHASE AC MOTOR INVESTIGATION:

The specifications of the motor described on the name plate are given below;

Manufacturer: Gryphon Brook Motor Limited
Horsepower output: 0.166 HP
Time rating: Continuous
RPM at rated load: 2800 rmp
Frequency: 50 Hz
Number of phases: Single Phase AC Motor
Rated load current: 1.5 amps
Voltage Rating: 220-240 volts

The above discussed single AC motor does not have any capacitor, it has only start and run windings and both are energised when the motor is started. These types of motors are known as “Single Phase Split Phase AC motors” and mostly used in medium starting applications.

Single phase induction motors are similar to 3-phase induction motors. They are composed of a squirrel -cage rotor (identical to that in 3-phase motor) and a stator. The stator carries a main winding, which carries a set of North and South Poles, it also carries; a smaller Auxiliary winding that only operates during the brief period of time when the motor stats up . The numbers of pole are same in both windings.

STATOR:

The stator of the motor consists of a steel frame that supports a hollow, cylindrical core made up of laminations. A number of evenly spaced slots, punched out of the internal circumference of the laminations, provide the space for the stator winding.

The figure 3 reveals the inner hollow, cylindrical core of the motor. The stator of this typical single phase AC motor consists of 18 evenly spaced slots.

The figure 4 shows the top view of the motor where the stator windings are pushed into the slots, start or auxiliary windings can also be seen connected to the centrifugal switch.

ROTOR:

A cage rotor is composed of bare copper bars, slightly longer than the rotor, which are pushed into the slots. The opposite are welded to two copper end-rings, so that all the bars are short circuited together. The entire construction bars and end rings resemble a cage which the name is derived.

The bearings and spring attached to the centrifugal switch are mounted on the end of the rotor

CENTRIFUGAL SWITCH:

Figure 7 shows, A split-phase / cage rotor single phase motor equipped with auxiliary winding to start the motor turning. The auxiliary windings often called the start winding must be disconnected from the main stator windings before the motor reaches nominal speed. This typical function is accomplished by a speed governor which is a centrifugal switch. The typical design calls for the start-winding circuit to be cut-out between 75% and 80% of the nominal speed of the motor.

The centrifugal switch consists of two parts; the first part of the switch is a centrifugal mechanism rotates on the motor shaft and the second part is a fixed stationary switch with electrical contacts which interacts with the first part to control the start winding circuit.

COOLING FAN:

This typical AC motor is TEFC enclosed which means, “Totally Enclosed, Fan Cooled” This is probably the most commonly used motor in ordinary industrial environments. The enclosure type offers good protection against common hazards. It is also constructed with a small fan on the rear shaft of the motor. This fan draws air over the motor fins, removing excess heat and cooling the motor. The enclosure is "Totally Enclosed". This ordinarily means that the motor is dust tight, and has a moderate water seal as well.

The heat is removed from the machine by the fan blades attached to the one end-ring of the rotor. The forced air travels axially along the machine exterior, which has fins to increase the dissipation.

4.7 INDUCTION MACHINE EQUIVALENT CIRCUIT:

The induction machine equivalent circuit is that of the transformer, as the induction machine has balanced windings. The pattern of the connection doesn't matter whether it is star or delta connected, because for the equivalent circuit calculation only one phase calculations are required.

To simplify calculations, the approximate equivalent circuit is used in which the air gap between stator and rotor is removed; the error is in the order of less than 10%.

All circuit values are referred to the stator as it is not feasible to connect the instruments to a secondary windings or a caged rotor.

The values of R1 and R2 represent the electrical losses in the stator and in the rotor, respectively. For a well-designed machine, the magnetic core loss must be equal to the copper/conductor loss. To measure and calculate the parameters of the equivalent circuit, few test are recommended such as;

The induction motor no-load test, which is equivalent to the transformer open circuit test and voltage, current and power are measured without any mechanical load. This determines the values for Rm and jXm.

The induction motor locked rotor test is equivalent to the transformer short circuit test. The rotor is clamped or locked so that it is unable to rotate and again the voltage, power and current are measured.

The resistance of the stator R1 was measured as 15.52 Ω by connecting a multi-meter across the rotor.

4.8 ROTOR SPEED AND SLIP:

The slip is defined as the ratio of the rotating magnetic field sweeping pass the rotor and the synchronous speed of the stator magnetic field, which can be written as;

S = (Ns - Nr)/Ns = 1- NrNs

In this typical case, the slip would be calculated as;

RMP speed = 2800 for a 2 pole motor at frequency 50 Hz

Synchronous speed = Ns = 120 x (50/2) = 3000 RPM

So, slip = S = 1- NrNs = 1- (2800/3000) = 0.06

In an induction motor slip is always positive, but in a generating function slip therefore would be positive. In both, the motoring mode and the generating mode, higher rotor slips induce higher current in the rotor and higher electromagnet conversion. Higher slip results in extra power loss by means of high rotor temperature.

4.9 ROTOR RESISTANCE EFFECT:

The resistance of the rotor of a squirrel cage motor is normally remains constant from no load to full load. The rotor resistance is subject to change with temperature in the induction machine during operation. So, he only factor which disturbs the resistance of the rotor is temperature, which increases with the increase in the load. Thus, the resistance increase with increasing load because the temperature rises.

The resistance of the rotor can be varied by using the wide range of the copper, aluminium or other metals in the rotor bars and end-rings. The torque-speed curve is affected by a great value by such a change in resistance.

In summary, the speed of the rotor decrease much less with increasing load under a low rotor resistance. Consequently, the efficiency of the rotor will be high and the motor tends to run cool.

4.10 INDUCTION MOTOR AS A GENERATOR:

4.10.1 INTRODUCTION:

The section of this chapter will include the brief induction to generator technology, and later in this chapter I will build and operate an induction motor as an induction generator to generate the electrical power.

4.10.2 AIMS AND OBJECTIVES:

The basic aim of this chapter is to carry out an investigation through the subject of induction machine followed by operating an induction motor as a generator. The operation includes the gain of a clear sine wave on oscilloscope.

4.10.3 INDUCTION GENERATORS:

Inelectricity generation, anelectric generatoris a basic electric device that convertsmechanical energytoelectrical energy. The reverse conversion of electrical energy into mechanical energy is done by amotor. Aninduction generatororasynchronous generatoris a type of ACelectrical generator, which functions on the principles of the induction motor to generate electrical power. Induction generators generate electric power when their rotor is rotated using an external mechanical power. In most case, a regular AC asynchronous motor can be used as an induction generator, without any internal modifications.

Induction generator topology is very much based on the relatively mature electric motor technology. Induction generators are perhaps the most common kinds of electric motors used in present era throughout the industry. When the suitable power supplies were not available, the induction generators were developed by using the fixed capacitor. Consequently, these generators resulted in low performances because of the unstable power output. In recent years, approach to the distributing generation system and availability of high power which gives the adjustable excitation and operate in stable manner with appropriate controls.

4.10.4 PRINCIPLE OF OPERATION:

Induction machines (motors and generators), generates electric power when their rotating segment (shaft, rotor) is rotated faster than the synchronous speed. For a typical two-pole motor which has one pair on stator running on a 60 Hz electrical grid will have a synchronous speed of 1800 revolution per minute (RMP). The same induction motor with two poles will have different synchronous speed of 1500 RMP operating at 50 Hz. As synchronous speed is given by the formula;

Synchronous speed = 120 x fp

Where f = frequency and p = number of poles

Furthermore, the induction machines operates as a generator when the rotor of the induction motor is driven at super synchronous speed, which is normally greater than 50 rev/sec or 1800 RPM for a two pole machine, which gives the reverse power flow of machine.

When the rotor speed is faster than the synchronous stator speed, it results in a negative slip. Consequently, the induced rotor currents produce a force on the conductors in the opposite direction to which it is being rotated; a mechanical input force is required to tackle the reverse torque and the mechanical power is converted to electrical output power in the stator. There is some power loss in the form of iron and copper loss. In short, motor is now operating as a generator, and sending power back to the electrical grid.

4.10.5 SELF-EXCITED GENERATION:

There is no need of connection to the grid while operating the induction generator. However, very large shunt capacitance is required to excite the generation. This large capacitance is connected to the stator windings/terminals; it supports to supply the lagging stator current.

One of the disadvantages of introducing the shunt capacitance is that it is not economically practical. Furthermore, to maintain a constant output frequency the input (mechanical input) speed needs to be constant. This system depends upon residual magnetism in the rotor to start generating.

4.10.6 EQUIPMENT FOR EXPERIMENT:

The following equipment will be used to carry out this operation;

  • Single phase AC motor
  • DC motor
  • High voltage AC power supply
  • Running capacitors 2 x 330uF
  • Digital oscilloscope
  • Multi Meter
  • Anemometer

4.10.7 EXPERIMENTAL PROCEDURE:

  • The following were the key step involved in conversion of an induction motor to a generator; Conversion of induction motor as generator

Steps Involved:

  • Electrical connection to the main supply 220-240 volts
  • Connection to the DC motor to the Mechanical Output i.e. turns the rotor above the synchronous speed.
  • Introducing a run capacitor into the connection and Running the motor without load and check the output power by connecting an oscilloscope to the stator terminals of motor
  • Testing of output power by means of output voltage regulation which depends on the speed of the motor

The typical induction motor for this lab used to generate electric power has specifications as below;

Manufacturer: Gryphon Brook Motor Limited
Horsepower output: 0.166 HP
Time rating: Continuous
RPM at rated load: 2800 rmp

Number of poles = 2 poles
Frequency: 50 Hz

Number of phases: Single Phase AC Motor
Rated load current: 1.5 amps
Voltage Rating: 220-240 volts

The given induction motor was disassemble and investigated before it was used to generate the electric power. The detailed investigation of the induction motor has been described in the previous chapter.

A DC motor was joined with the single phase AC motor to provide the mechanical input power to the generator to rotate its shaft. It has following specifications;

Output power = 0.25 Kw

Type = PM (permanent magnet)

Speed = 2000 RPM

Volts = 24 volts

Amps = 14 amps

Both motors were firmly attached to the pallet of a steel to avoid any kind of movement while they are operating.

The induction generator will operate without connection to the electric grid; however, very large shunt capacitance has introduced to supply the lagging stator current by means of two running capacitors of each value of 330uF and connected to the stator terminals.

The DC motor is energised with high voltage which resulted in the rotation of its shaft. Consequently, the shaft of DC motor gives mechanical input power to the attached rotor of induction machine.

When the rotor of induction motor is rotated over the synchronous speed, the induced rotor currents produce a force on the conductors in the opposite direction in which it is driven. The speed of the rotor was controlled by varying the input voltage and speed of the DC motor.

The input voltage was altered at different points and the speed of the rotor was measured using a device called anemometer. This is the synchronous speed of the induction motor. The oscilloscope was connected to the stator winding and a clear sinusoidal waveform was noticed which indicates the generation of the AC voltage or electric power.

The generation of the electric power can be justified as, if the induction motor is driven faster than the synchronous speed, electric power is generated utilizing the mechanical input power from the DC motor or a prime mover such as gas, wind in case of renewable system. The generated power is a function of the slip, and varies with the slip itself.

Single phase electric motor follows the direction in which they are energised and they can be reconnected in the other way to reverse the direction of the rotation. The normal ac motor can be run in any direction but induction motor for the use of a generator is anti-clockwise direction when facing the end opposite the drive shaft.

4.10.7 EXPERIMENT RESULTS:

To calculate thesynchronous speed of an induction motor, we use the following formula:

Synchronous speed =120 x f/p = 120 x 50/2 = 3000 RMP

The slip of the induction machine was made negative by running the motor faster than synchronous speed. This was measured as 3200 RPM, so the slip of the machine can be calculated as;

S = (NS -Nr)/ NS = (3000-3200)/ 3000 = -0.06

The value of R2/s in the equivalent circuit is, therefore,

R2/s = 15.52/-0.06 = -258 Ω

The negative resistance indicates that power is flowing from the rotor to the stator rather from the stator to the rotor.

The Full-loadTorque or simply braking torque is the torque required to produce the rated power of the electrical motor at full-load speed or Braking torque is the maximum torque that the motor develops at rated voltage and frequency, without an abrupt drop in speed. And this can be represented by the equation,

T =5252 x Php/ Nr (1)

Where,

T= braking torque (lb ft)

Php= rated horsepower

Nr= rated rotational speed (rev/min, rpm)

So, the braking torque for this generator can be calculated as

T = 5252 x .166/2800 = 0.311 lb ft

4.10.8 TORQUE VS SPEED CHARCATEISTICS:

Induction motors function generally as generators if run above a critical speed or named RPM. Some source of electricity is required to energise the coils; this goal is normally achieved by the running capacitors to power the coils, or provided by external power supplies such as from a voltage source.

By varying the slip of the induction machine over a wide range in equivalent circuit, we obtain the torque speed characteristics

When the slip of the machine is negative, it operates as a generator to generate the power.

speed of the rotor (RPM)

voltage across the stator terminals (mV)

300

37

500

74

1150

185

1550

207

The above table and graph represent the relationship between the input speed of the rotor and the generated voltage. The higher the speed of the rotor will result in high generation of power in a suitable region.

4.10.9 GENERTION OF SINOUSIDAL WAVEFORM:

An induction generator follows the principle of faraday's law of electromagnetic induction to convert mechanical energy such as rotation in this case into electrical energy. The produced energy is the form of sinusoidal waveform.

The above diagrams depicts the operation of an electric induction motor which gives a clear output sinusoidal waveform when operated at higher speed than the synchronous speed i.e. when the slip of the machine is forced to be negative.

4.10.10 DISCUSSION:

The above discussed experiment converts an induction motor to an induction electric generator. The results achieved from the experiments are reliable and satisfactory

The induction motor is a constant speed motor when they are operated at constant input voltage and operating at constant frequency. When the load torque is changed the speed of the induction motor alters by a small percentage of the rated speed. The speed can be controlled by applying few procedures such as;

  • The speed of the induction motor can be changed over a small range for a given load by varying the line voltage.
  • By varying the applied frequency, which changes the synchronous speed of the motor.
  • By changing the stator winding connections, the total number of poles can be modified which results in different variation speed.

In an induction generator, when generators are operated at stand-alone system, the magnetizing flux to magnetise the rotor is supplied by acapacitor bankconnected to the stator windings of the machine and in case of grid connection it draws magnetizing current from the grid. For stand-alone systems, frequency and voltage are complex function of machine parameters, capacitance used for excitation, and load value and type.

Induction generators are mostly used inwind turbinesand somemicro hydroinstallations due to their ability to produce useful power at different speed of rotor. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, and do not come with brushes. Induction generators are very suitable and usually used for wind generating stations as in this case speed is always a variable factor, and the generator is easy on the gearbox.

4.11 CONCLUSION:

Using induction motors as generators is a very cost effective way of providing a generator for a turbine system.In single phase operations, it is possible to utilize induction motors as generators and the operation of the generator results in very good efficiency, when they are connected to the running capacitors. The use of the capacitors results in smooth running of the generator, and functioning at 100 % power factor (PF) in some case. The efficiency can be increased by the generator running balanced on all three legs of the motor.

4.12 SUMMARY:

The chapter dealt with the study of the induction machine and more specifically the single phase AC motor, which was then experimented to function as induction generator to generate power. Furthermore, Dynamics of self-excitation process of an induction generator, using a fixed capacitor bank, have been investigated experimentally for a single cage induction machine.

4.13 Achievements and Future Work

The aims and objectives of the proposed work were achieved successfully. The whole thought of the work was to have a better understanding of the electrical machines especially induction motors and induction machines.

The project work can be carried into the large scale, where the whole renewable system is involved. The future work can also be carried out into the wind power system where the whole systems work as a unit.