Staebler Wronski Effect

When light strikes on hydrogenated amorphous silicon it exhibits a metastable change in the photovoltaic properties. When hydrogenated amorphous silicon is exposed prolonged to the light its efficiency to produce electricity decreases that is called Staebler Wronski Effect. This degradation of the electrical output of hydrogenated amorphous silicon is due to prolonged illumination. David L. Staebler and Christopher R. Wronski discovered this fact in 1977. The intensity of degradation depends on the diffusion coefficient of hydrogen and local bonding formation promoted by hydrogen.

Explanation of Staebler-Wronski Effect

The Stabler Wronski theory suggested that when intense light falls on hydrogenated amorphous silicon, the electron-hole pairs are created which again combine with neighboring Si-Si bonds that have weak foundationDuring recombination process the tremendous amount of energy is released which creates defects and causes degradation noncrystalline structure of the hydrogenated amorphous silicon. The recombinations occur especially in those regions where the band edges are very close to one another energetically and also have the smaller concentration of hydrogen. The breakage leads to enhanced hydrogen diffusion, and the creation of dangling bonds (H atom then forms a new bond with neighboring Si atom), and this reduces the flow of current by tapping electron-hole pairs. The movement of hydrogen atoms is responsible for the creation of dangling bonds. This phenomenon in hydrogenated amorphous silicon reduces the conversion of light energy into electrical energy. The Staebler Wronski effect reduces the efficiency of solar cell up to 15 % within the first 1,000 Hrs.

Factors which affects Staebler-Wronski Effect

• Hydrogen Concentration - Hydrogen concentration plays an important role in Staebler-Wronski Effect, which breaks further bonds and causing dangling bonds which obstruct flow of electrons.
• The Physical State of Silicon - It has been noted that hydrogenated amorphous silicon is more suitable for making the solar cell than nanocrystalline silicon. Amorphous silicon contains the substantial quantity of hydrogen which passivates Si dangling bonds and results in an improved electrical property. But it has the property of forming Si-H bond which degrades upon light soaking and attends a metastable state and this disorder of amorphous silicon suffers more from Staebler-Wronski effect.
• Concentration of Impurities - The efficiency of hydrogenated amorphous silicon depends on the material quality. Staebler-Wronski effect is associated with the concentration of impurities in the material and this effect causes blocking in annihilation. But now this theory does not hold good as it is well established by experiments that best quality of hydrogenated amorphous silicon (a-Si:H) has an inhomogeneous distribution of hydrogen and hydrogen increases the band gap in hydrogenated amorphous silicon, which creates density fluctuations.
• Temperature - The Staebler-Wronski effect is more effective below 400^ok. But at high-temperature Staebler-Wronski effect reduces drastically. Heating the material above 200^oC can anneal back the effect. After a sustained temperature, breakage by bond-pair is limited and healing beats breakages and recovery starts from there. The Staebler-Wronski effect disappears when the band gap falls below the critical level.

Methods to Overcome Staebler-Wronski Effect

• Use of polycrystalline, multi-crystalline or single-crystal silicon in place of amorphous silicon as they show more stability than a-Si.
• Working at high temperature as this effect disappears or becomes weak at high temperature.
• Use multijunction solar cell. In this cell, thin layers of amorphous silicon are stacked with some other materials to form a solar cell. When intense electric field passes through these amorphous silicon layers, the effect of Staebler-Wronski get reduced.

  Paramagnetic Properties of Amorphous Silicon

  The metastable defect of Amorphous Silicon shows paramagnetic properties that vary with respect to the microscopic environment. These defects of a-Si actually come in two types. One is uniformly distributed and other is concentrated in clusters within the internal surface of the cell during the manufacturing process. The clusters of defects are generated on the internal walls of microvoids which contribute to the light-induced degradation of amorphous silicon in the thin film solar cell.

What is Photovoltaic Effect?

The effect due to which light energy is converted to electric energy in certain semiconductor materials is known as photovoltaic effect. This directly converts light energy to electricity without any intermediate process. For demonstrating the photovoltaic effect let us assume a block of silicon crystal.
The upper portion of this block is doped with donor impurities and lower portion is doped with accept or impurities. Hence the concentration of free electrons is quite high in n – type region compared to p-type region and concentration of hole is quite high in p-type region compared to n-type region of the block. There will be a high concentration gradient of charge carriers across the junction line of the block. Free electrons from n-type region try to diffuse to p-type region and holes in p-type region try to diffuse to n-type region in the crystal. This is because charge carriers by nature always tend to diffuse from high concentration region to low concentration region. Each free electron of n-type region while comes to the p-type region due to diffusion, it leaves a positive donor ion behind it in the n-type region.This is because each of the free electron in n-type region is contributed by one neutral donor atom. Similarly when a hole is diffused from p-type region to n-type region, it leaves a negative accept or ion behind it in p-type region.
Since each hole is contributed by one acceptor atom in p-type region. Both of these ions i.e. donor ions and acceptor ions are immobile and fixed at their position in crystal structure. It is needless to say that those free electrons of n-type region which are nearest to the p-type region first diffuse in the p-type region consequently create a layer of positive immobile donor ions in the n-type region adjacent to the junction.Similarly those free holes of p-type region which are nearest to the n-type region first diffuse in the n-type region consequently create a layer of negative immobile acceptor ions in the p-type region adjacent to the junction. These positive and negative ions concentration layer creates an electric field across the junction which is directed from positive to negative that in from n-type side to p-type side. Now due to presence of this electric field charge carriers in the crystal experience a force to drift according to the direction of this electric field. As we know the positive charge always drift in the direction of electric field hence the positively charged holes (if any) in n-type region now drift to the p-side of the junction.On the other hand, negatively charged electrons in p-type region (if any) drift to n-region as negative charge always drift opposite to the direction of electric field. Across a p-n junction diffusion and drift of charge carriers continues. Diffusion of charge carriers creates and increases the thickness of the potential barrier across the junction and drift of the charge carriers reduces the thickness of the barrier. In normal thermal equilibrium condition and in absence of any external force, the diffusion of charge carrier is equal and opposite of drift of charge carriers hence the thickness of potential barrier remains fixed.

Now the n-type surface of the silicon crystal block is exposed to the sunlight. Some of the photons are absorbed by the silicon block. Some of the absorbed photon will have energy greater than the energy gap between valence and conduction band of valence electrons of the silicon atoms. Hence, some of the valence electrons in the covalent bond will be excited and jump out from the bond leaving behind a hole in the bond. In this way electron-hole pairs are generated in the crystal due to incident light. The holes of these light generated electron-hole pairs in the n-type side have enough probability of recombination with enormous electrons (majority carriers). Hence, solar cell is so designed, that the light- generated electrons or holes will not get enough chances to recombine with majority carriers.

The semiconductor (silicon) is so doped that the p-n junction forms in very close vicinity of exposed surface of the cell. If an electron hole pair is created within one minority carrier diffusion length, of the junction, the electrons of electron-hole pair will drift toward n-type region and hole of the pair will swept to p region due to in influence of electric field of the junction and hence on the average, it will contribute to current flow in an external circuit.

Components of a Solar Electric Generating System

Solar Panels

The main part of a solar electric system is the solar panel. There are various types of solar panel available in the market. Solar panels are also known as photovoltaic solar panels. Solar panel or solar module is basically an array of series and parallel connected solar cells. The potential difference developed across a solar cell is about 0.5 volt and hence desired number of such cells to be connected in series to achieve 14 to 18 volts to charge a standard battery of 12 volts. Solar panels are connected together to create a solar array. Multiple panels are connected together both in parallel and series to achieve higher current and higher voltagerespectively.

Batteries

In grid-tie solar generation system, the solar modules are directly connected to inverter not with load. The power collected from solar panel not in constant rate rather it varies with intensity of sunlight. This is the reason why solar modules or panels do not feed any electrical equipment directly instead they feed an inverter whose output is synchronized with external grid supply. Inverter takes care of the voltage level and frequency of the output power from the solar system it always maintains it with that of grid power level. As we get power from both solar panels and external grid power supply system, the voltage level and quality of power remain constant. As the stand-alone or grid fallback system is not connected with grid any variation of power level in the system can directly affects the performance of the electrical equipment fed from it. So there must be some means to maintain the voltage level and power supply rate of the system. A battery bank connected parallel to this system takes care of that. Here the battery is charged by solar electricity and this battery then feeds a load directly or through an inverter. In this way variation of power quality due to variation of sunlight intensity can be avoided in solar power system instead an uninterrupted uniform power supply is maintained. Normally Deep cycle lead acid batteries are used for this purpose. These batteries are typically designed to make capable of several charging and discharging during service. The battery sets available in the market are generally of either 6 volt or 12 volts. Hence number of such batteries can be connected in both series as well as parallel to get higher voltage and current rating of the battery system.

Controller

This is not desirable to overcharge and under discharge a lead acid battery. Both overcharging and under discharging can badly damage the battery system. To avoid these both situations a controller is required to attach with the system to maintain flow of current to and fro the batteries.

Inverter

It is obvious that the electricity produced in a solar panel is DC. Electricity we get from the grid supply is AC. So for running common equipment from grid as well as solar system, it is required to install an inverter to convert DC of solar system to AC of same level as grid supply. In off grid system the inverter is directly connected across the battery terminals so that DC coming from the batteries is first converted to AC then fed to the equipment. In grid tie system the solar panel is directly connected to inverter and this inverter then feeds the grid with same voltage and frequency power.In modern grid tie system, each solar module is connected to grid through individual micro-inverter to achieve high voltage alternating current from each individual solar panel.

Components of Stand Alone Solar System

A basic block diagram of a stand-alone solar electric system is show above. Here the electric power produced in the solar panel is first supplied to the solar controller which in turn charges the battery bank or supplies directly to the low voltage DC equipments such as laptops and LED lighting system. Normally the battery is fed from solar controller but it can also feed the solar controller when there is insufficient supply of power from solar panel. In this way the supply is continued uniformly to the low voltage equipments which are directly connected to the solar controller. In this scheme the battery bank terminals are also connected across an inverter. The inverter converts the stored DC power of the battery bank to high voltage AC for running larger electrical equipments such as washing machines, larger televisions and kitchen appliances etc.

Components of Grid Tie Solar System

Grid tie solar systems are of two types one with single macro central inverter and other with multiple micro inverters. In the former type of solar system, the solar panels as well as grid supply are connected to a common central inverter called grid tie inverter as shown below.The inverter here converts the DC of the solar panel to grid level AC and then feeds to the grid as well as the consumer’s distribution panel depending upon the instantaneous demand of the systems. Here grid-tie inverter also monitors the power being supplied from the grid. If it finds any power cut in the grid, it actuates switching system of the solar system to disconnect it from the grid to ensure no solar electricity can be fed back to the grid during power cut. There is on energy meter connected in the main grid supply line to record the energy export to the grid and energy import from the grid.
As we already told there is another type of grid-tie system where multiple micro-inverters are used. Here one micro inverter is connected for each individual solar module. The basic block diagram of this system is very similar to previous one except the micro inverters are connected together to produce desired high AC voltage.In previous case the low direct voltage of solar panels is first converted to alternating voltage then it is transformed to high alternating voltage by transformation action in the inverter itself but in this case the individual alternating output voltage of micro inverters are added together to produce high alternating voltage.

Types of Solar Power Station

There are mainly four types of solar power stations.

1. Stand Alone or Off Grid type Solar Power Plant
2. Grid Tie type Solar Power Plant
3. Grid Tie with Power Backup or Grid Interactive type Solar Power Plant
4. Grid Fallback type Solar Power Plant.

Let us discuss a brief introduction of each type of solar power plant.

Stand Alone or Off Grid Solar Power Station

This is most commonly used photo-voltaic installation used to provide localized electricity in absence of conventional source of electric power at certain location. As the name prefers this system does not keep any direct or indirect connection with any grid type network.
In standalone system the solar modules produce electric energy which is utilized to charge a storage battery and this battery delivers electricity to the connected load. Standalone systems are normally small system with less than 1 kilo watt generation capacity.

Grid Tie Solar Power Station

In some countries facility is available of selling power to the local or national grid. This is gaining popularity in Europe and the United States. This system facilitates both electric utility companies as well as the consumers. Here consumers can generate electricity by their own plant and can sell the surplus to the electricity utility company through grid connected to their plant. As the consumers sell the power they can earn money as return of their investment for installation of captive power plant on the other hand electric utility companies can reduce their capital investment on their own plant for power generation. In a grid-tie solar system, consumers consume electricity produced by solar captive power plant during sunny day time and also export surplus energy to grid but at night while solar plant does not produce energy, they import electric energy from grid for consumption. The main disadvantage of this system is that if there is a power cut in the grid, the solar modules should be disconnected from the grid. This system is not always very profitable especially where overall maximum demand of the system does not occur at the peak sunny period of the day. In hot climate where the power demand for air conditioning machines becomes maximum during peak sunny period of the day, this grid tie solar power generation system works most efficiently.
Grid tie solar systems are of two types one with single macro central inverter and other with multiple micro inverters. In the former type of solar system, the solar panels as well as grid supply are connected to a common central inverter called grid tie inverter as shown below. The inverter here converts the DC of the solar panel to grid level AC and then feeds to the grid as well as the consumer’s distribution panel depending upon the instant demand of the systems. Here grid-tie inverter also monitors the power being supplied from the grid. If it finds any power cut in the grid, it actuates switching system of the solar system to disconnect it from the grid to ensure no solar electricity can be fed back to the grid during power cut. There is on energy meterconnected in the main grid supply line to record the energy export to the grid and energy import from the grid.

As we already told there is another type of grid-tie system where multiple micro-inverters are used. Here one micro inverter is connected for each individual solar module. The basic block diagram of this system is very similar to previous one except the micro inverters are connected together to produce desired high AC voltage. In previous case the low direct voltage of solar panels is first converted to alternating voltage then it is transformed to high alternating voltage by transformation action in the inverter itself but in this case the individual alternating output voltage of micro inverters are added together to produce high alternating voltage.

Grid Tie with Power Backup Solar Power Generation

It is also called grid interactive system. This is a combination of a grid-tie solar power generation unit and storage battery bank. As we said, the main drawback of grid tie system is that when there is any power cut in the grid the solar module is disconnected from the system. For avoiding discontinuity of supply during power cut period one battery bank of sufficient capacity can be connected with the system as power backup.

Grid Fallback Solar Power Generation

Grid fallback is most reliable and stable system mainly used for electrifying smaller households. Here solar modules charge a battery bank which in turn supplies distribution boards through an inverter. When the batteries are discharged to a pre-specified level, the system automatically switches back to the grid power supply. The solar modules then recharge the batteries and after the batteries are being charged up to a pre-specified level again the system switches back to solar power. We do not sell electricity back to the electricity utility companies through this system. All the power that we produce is utilized for ourselves only.
Although we do not have any direct earning benefit from this system but the system has its own big advantages. This system is most popular where there is no facility of selling power to the grid.
Grid fallback system has all advantages of grid interactive system except power selling, but it adds benefit of using own power whenever it is required irrespective of position and condition of sun in the sky.

Solar Energy System | History of Solar Energy

Solar Energy System

Solar energy is the light and radiant heat from the Sun that control Earth's climate and weather and protract life. It is a renewable source of energy and originates with the thermonuclear process that transfers about 650,000,000 tons of hydrogen to helium per second. This action produces lots of heat and electromagnetic radiation. The produced heat remains in the sun and is helpful in upholding the thermonuclear reaction and electromagnetic radiation together with visible, infrared and ultra-violet radiation flow out into space in all directions. Solar energy is in reality nuclear energy. Similar to all stars, the sun is a large gas sphere made up mostly of hydrogen and helium gas. In the internal surface of sun 25% of hydrogen is fusing into helium at a rate of about 7 × 10¹¹ kg of hydrogen per second.

Heat from the center is first and foremost spread out, and then sends down, to the Sun surface, where it keeps up at a temperature of 5800 K. According to Stefan-Boltzmann’s Law, the total energy that is released by the Sun, and therefore, the quantity of solar energy that we get here on Earth, is significantly reliant upon this surface temperature. Now a day’s solar energy system play an important role in the field of producing electricity or other domestic uses like water heating, cooking etc. As we know that major part of generated electricity or electricity depends upon coal which is used in thermal power plant (in In India 65% of total power is generated by the thermal power plant). But the main problem is here that the fuel used in thermal power plant is coal which is in limited amount and may be not available in future to produce or generate electricity. That is the main reason to solar energy system comes to the picture.

Solar energy system is the pollution free source of energy and always available because, sun is the single source of solar energy (also known as renewable energy or non conventional energy) which sits at the central point of solar system and radiate energy at an tremendously huge and fairly constant rate, per day per year as the form of electromagnetic radiation. Sun contained huge amount of energy but the whole energy not utilized at earth due to some reason like-

• Earth is revolve at about its polar axis.
• Atmospheric reason of earth.
• Earth is relocate from the sun.

But the main thing is that after these obstacles sun energy reach to earth is sufficient to produce or generate electricity which is polluted free. Due to this consideration somewhat we reduce the use of Thermal Power Plant, Gas Power Plant etc and reserve the non renewable energy sources like coal, petroleum etc for future. In recent years solar energy system is rise as a primary source of energy that converted into electricity and almost all country in the world utilize maximum solar energy to produce electricity and this is very less costly.
The main benefit of solar energy system is that the sun light is present everywhere free of cost. To produce electricity or convert solar energy to other form of energy first we invest heavy amount for solar panel that convert solar energy to other form but the main advantage is after that installation no any type of maintenance is required for 40 to 50 years.

History of Solar Energy

First solar collector created by Swiss scientist named Horace-Benedict de Saussure in 1767, he take an insulated box enclosed with three layers of glass which suck up heat energy. After that Saussure’s box became famous and widely known as the first solar oven, getting temperatures of 230 degrees Fahrenheit. After that in 1839 a most important landmark in the progression of solar energy occurs with the significant of the photovoltaic effect by a French scientist Edmond Becquerel. In this he used two electrodes placed in an electrolyte and then exposing it to the light and results is tremendous electricity increased a lot. After that lots of experiment are occurred by various scientists at time to time and modified our solar energy system to produce more electricity from solar energy. But now a day’s also in this field various experiments are doing by a scientist, how to utilize maximum solar energy which is available on the earth.

In 1873, Willoughby Smith discovered photoconductivity of a material known as selenium. In 1887 there was the discovery of the ultraviolet ray capacity to cause a spark jump between two electrodes and this was done by Heinrich Hertz. In 1891 the first solar heater was created. In 1893 the first solar cell was introduced. In 1908 William J. Baileys invented a copper collector which was constructed using copper coils and boxes. In 1958, solar energy was used in space. In the 1970′s, Exxon Corporation designed an efficient solar panel which was less costly to manufacture. Less cost manufacturing process of solar panel became the major milestone in the history of solar energy. In 1977 the US government embraced the use of solar energy by launching the Solar Energy Research Institute. In 1981, Paul Macready produced the first solar powered aircraft. in the year 1982 there was the development of the first solar powered cars in Australia. In 1999 the largest plant was developed producing more than 20 kilowatts.

In 1999, the most proficient solar cell was developed with a photo-voltaic efficiency of 36 percent, now a day we produce 200 megawatts to 600 megawatts electricity from solar energy like in India’s Gujarat Solar Park, a compilation of solar farms spotted around the Gujarat region, show a mutual installed capacity of 605 megawatts and Golmud Solar Park in China, with an installed capacity of 200 megawatts.

Solar Electricity

The electricity produced by direct hitting of sunlight on photo - voltaic cells, is called solar electricity.

Solar Electricity

When sunlight strikes on photo-voltaic solar cells, solar electricity is produced. That is why this is also referred to as Photo Voltaic Solar, or PV Solar.

Principles of Solar Electricity

Generation of electricity by using solar energy depends on the photo voltaic effect. In photo voltaic effect, semiconductor p n junction produces electric potentia when it is exposed to sunlight. For that purpose, we make n type semiconductor layer of the junction very thin. It is less than 1 µm thick. The top layer is n layer. We generally refer it as emitter of the cell.
The bottom layer is p type semiconductor layer and it is much thicker than top n layer. It may be more than 100 µm thick. We call this bottom layer as base of the cell. The depletion region is created at the junction of these two layers due to immobile ions.

When sunlight strikes on the cell, it easily reaches up to p n junction. The p n junction absorbs the photons of sunlight ray and consequently, produces electrons holes pairs in the junction. Actually, the energy associated with photon excites the valence electrons of the semiconductor atoms and hence the electrons jump to the conduction band from valence band leaving a hole behind each.The free electrons, find themselves in the depletion region will easily pass to the top n layer because of attraction force positive ions in the depletion region. In the same way the holes find themselves in the depletion region will easily pass to the bottom p layer because of attraction force of negative ions in the depletion region. This phenomenon creates a charge difference between the layers and resulting to a tiny potential difference between them.The unit of such combination of n type and p type semiconductor materials for producing electric potential difference in sunlight is called solar cell. Silicon is normally used as the semiconductor material for producing such solar cell.Conductive metal strips attached to the cells take the solar cell or photo voltaic cell is not capable of producing desired electricity instead it produces very tiny amount of electricity. Hence for extracting the desired level of electricity required numbers of such cells are connected together in both parallel and series to form a solar module or photo voltaic module. Actually, only sunlight is not the factor. The main factor is light or beam of photons to produce electricity in the solar cell. Hence a solar cell can also work in cloudy weather as well as in moonlight but then electricity production rate becomes law as it depends upon the intensity of incident light ray.

Application of Solar Electricity

Solar electric power generation system is useful for producing moderate amount of power. The system works as long as there is a good intensity of natural sunlight. The place where solar modules are installed should be free from obstacles such as trees and buildings otherwise there will be the shade on the solar panel which affects the performance of the system. It is a general view that solar electricity is an impractical alternative of the conventional source of electricity and should be used when there is no traditional alternative of the conventional source of electricity available. But this is not the actual case. Often it seems that solar electricity is more money saving alternative than other traditional alternatives of conventional electricity.For examples : - It is always economical to install a solar light or a solar power source where it is difficult and costly to get point from local electric supply authority such as in remote garden, shed or garage where standard electric supply point is not available. The solar electricity system is more reliable and uninterrupted as it does not suffer from unwanted power cut from an electric supply company. For constructing a mobile electric power source, for moderate power requirements, the solar module is a good choice. It can be useful whilst camping, working on outdoor sites. It is most effective means of creating green energy for our own purpose and may be for selling surplus energy to customers but for producing electricity in commercial scale the investment and volume of the system become large enough.
In that case area of the project will be much larger than conventional one. Although for running few lights and low-power electrical gadgets such as laptop computer, portable sized television, mini fridge etc solar electricity system is quite suitable provided there is sufficient free space on ground or on the roof top for installing solar panels. But it is not at all economical to run high-power consuming electric pieces of equipment like high-speed fans, heaters, washing machines, air conditioners and power tools with the help of solar electricity as the cost of production such high energy is quite higher that it is expected. Moreover, there may be the lack of space availability in your premises for installation of a large solar panel.
Ideal uses of low-cost solar panels are charging batteries in caravans and recreational vehicles or on boats when these are not in movement provided there should be trickle charging facility from dynamo during movement of these vehicles.

Measurement of Insulation Resistance

We define insulation resistance as the ratio of applied direct voltage across an insulation to the corresponding current through it.

Mesaurement of inslation resistance is quite important. We normally take the reading of the measurement at a certain time after application of test voltage. The standard duration of voltage application are 1 minute or 10 minutes. Because of that, insulation resistance can also be referred as 1 minute insulation resistance or 10 minute insulation resistance depending upon the duration of the test.

NB: - The voltage, we apply for measurement of insulation resistance, is direct voltage. When we apply direct voltage across the insulation, a current starts passing through the insulation. This current has two main components.

1. The current flowing through the leakage path over the surface of the solid insulator. This leakage path is formed mainly due to moisture, dust etc. which are naturally accumulated on the surface of the solid insulator.
2. The current flowing through the volume of the insulator body.

The second component of the current is further divided in three components as mentioned below.

• As the insulation materials are essentially dielectric in nature, there will be a capacitive charging current, appears just after application of test voltage. This current is instantaneous in nature. It will effectively disappear within few moments. Hence, this current does not have any effect on the reading of measurement if it is taken after 1 minute or more.
• There is another component of current called absorption current. It decays from high value to zero. The insulation resistance value taken within first few minutes of test is largely dominated by absorption current.
• Last but most important component of the current is conduction current. It remains steady throughout the insulation resistance test. So after, charging current then absorption current becomes insignificant, the test result is mainly predominated by this conduction current.

Thus finally, leakage current and conduction current come into picture at the time of taking reading of insulation resistance. This is why the reading of insulation resistance is normally taken after 15 seconds or 1 minute or sometimes after 10 minutes during the test.

Method of Measuring Insulation Resistance

There are several instruments for measuring insulation resistance of an electrical equipment.

1. Direct-indicating ohmmeter with hand driven dc generator. This is locally known as hand driven megger since Megger is one of the best known manufacturer of this instrument.
2. Direct-indicating ohmmeter with motor driven dc generator. This is locally known as motorized megger.
3. Direct-indicating ohmmeter with self-contained battery.
4. Direct-indicating ohmmeter with self-contained rectifier. This instrument takes power from an external AC supply.
5. Resistance bridge circuit with self-contained galvanometer and battery.

We can conduct the measurement of insulation resistance with an external dc supply. In that case, we take voltage and current reading with the help of a dc voltmeter and a micro ranged dc ammeter, respectively.

In that case, we can calcula the insulation resistance with the help of ohm’s lawWhere V is the voltmeter reading and I is the ammeter reading.

The ammeter is micro ranged because, a very tiny current passes through the insulation during test and he current is in that range only. But at the moment of voltage application, the micrometer has to take initial capacitive charging current as well as absorption current. So the ammeter should be capable of withstanding both of these currents for at least initial duration. The voltmeter, ammeter and source should also be capable of withstanding short circuit current in the case of insulation failure if occurs during measurement.

When we use direct indicating ohmmeter of simply megger, the leads of the instrument are connected across the insulator to be tested. After driving the instrument the value of insulation resistance is indicated on the analog or digital dial of the instrument directly. In both of the above-mentioned methods of insulation resistance measurement, the reading is taken after a standard time delay to get more accurate and error-free reading.