The zero-emission power plant is no longer a fantasy. Filters are becoming increasingly sophisticated, removing dust and other harmful substances from exhaust gases. And in the future, new power generation technologies should prevent emissions of carbon dioxide getting into the atmosphere. Enhanced measurement methods and more efficient power distribution are also helping to ensure cleaner air.

Abstract

India is the world’s fourth largest economy and has a fast growing energy market. India’s current power capacity is 30% short of demand. Coal and petroleum are the primary sources of energy. High ash content in Indian coal and inefficient combustion technologies contribute to India’s emission of air particulate matter and other trace gases, including gases that are responsible for the greenhouse effect.

Climate change is one of the most serious single challenges faced by humankind today. Probably one of the greatest impacts in reducing CO2 emissions will be made by the introduction of Zero Emission Fossil Fuel Power Plants including carbon dioxide capture and storage.

Integrated Gasification Combined Cycle power plant

The Oxy-blown power plants are more compatible than our conventional power plants regarding on emission levels. Among them Integrated Gasification combined Cycle power plant is the best one.

Integrated Gasification Combined Cycle (IGCC) plants are already in operation. Here, a fuel such as coal is converted, with the addition of oxygen, into synthesis gas [Gasification]. Syngas is the primary fuel for IGCC applications. Natural gas is used for start up and as a backup fuel. During the start up process at 30% load, the gas turbine is transitioned to syngas and taken to base load and mainly carbon monoxide and hydrogen.

The gasification process can produce syngas from high-sulfur coal, heavy petroleum residues and biomass.

Figure 1 - Post-Combustion Capture Using Solvents

The plant is called integrated because its syngas is produced in a gasification unit in the plant which has been optimized for the plant’s combined cycle. In this example the syngas produced is used as fuel in a gas turbine which produces electrical power. To improve the overall process efficiency heat is recovered from both the gasification process and also the gas turbine exhaust in ‘Waste Heat Boilers’ producing steam. This steam is then used in steam turbines to produce additional electrical power.

The main feature is that instead of using oxygen and nitrogen to gasify coal, they use oxygen and CO2. The main advantage is that it is possible to improve the performance of cold gas efficiency and to reduce the unburned carbon (char).

• Proven lowest NOx, SOx, particulate matter and lower hazardous air pollutants,
• Proven mercury and carbon dioxide removal,
• Lower water usage, lower solids production
• Sulfur and non-leachable slag by-products

In an IGCC plant, CO2 can be separated during the synthesis gas preparation stage. However, separation compression and storage of the gas reduces the efficiency by about 12 percentage points. IGCC plants can also be fed biomass or wastes from the chemical industry such as asphalt and even car tires.

Mercury Emissions

IGCC is essentially the only coal technology that can effectively remove mercury from the environment. Carbon beds have demonstrated 99.9% mercury removal from syngas (post “gas-clean-up”).

Carbon Capture and Reduction Techniques

Before carbon dioxide (CO2) gas can be sequestered from power plants and other point sources, it must be captured as a relatively pure gas. CO2 is routinely separated and captured as a by-product from industrial processes such as synthetic ammonia production, H2 production, and limestone calcination.

There are three technology routes to capturing CO2: pre-combustion, post-combustion and oxyfuel combustion. post-combustion and oxyfuel combustion technologies that can be retrofitted to today’s coal plants. Both technologies are feasible, safe and have the potential to be cost-effective.

The challenge lies in developing the processes so that they can be deployed economically on a large scale.

CO2 Capture Options

CO2 capture involves the separation of CO2 from combustion gases and compressing it so that it is suitable for safe transport and storage. There are three basic capture systems to isolate CO2 from the combustion process: post-combustion separation, oxy-fuel firing, and pre-combustion separation.

Post-combustion Capture

In this process, the CO2 is separated from the flue gases after combustion has taken place. Instead of being discharged directly to the atmosphere, the flue gas is passed through an absorbent or a selective membrane, which separates most of the CO2.

The CO2, previously compressed, is fed to a storage reservoir and the remaining flue gas is discharged into the atmosphere.

Pre-combustion Capture

Pre-combustion capture involves reacting the fuel with oxygen or air, and possibly also with steam, to produce a ‘synthesis gas (syngas)’ or ‘fuel gas’, composed mainly of carbon monoxide and hydrogen. The carbon monoxide is then reacted with steam in a catalytic reactor, called a shift converter, to give CO2 and more hydrogen. Next, the CO2 is separated, usually by a physical or chemical absorption process, resulting in a hydrogen-rich fuel which can be used in many applications, such as boilers, furnaces, gas turbines, engines and fuel cells.

Oxy-fuel Firing

In oxy-fuel combustion, nearly pure oxygen is used for combustion instead of air, resulting in a flue gas that is mainly CO2 and H2O. This simplifies the separation process as the water vapour can readily be condensed to liquid, leaving the CO2 for subsequent treatment.

Carbon Control Technologies for Existing Plants

The several innovative schemes that could significantly reduce CO2 capture costs, compared to conventional processes.

These include:

1 Oxyfuel Combustion processes use oxygen rather than air for combustion. The oxy-fuel cycles are a promising technology. The combustion with pure oxygen leads to a working fluid consisting mainly of steam and CO2, which allows an easy and cost-effective CO2 separation by steam condensation. Further advantages are the great variety of fuels which can be used (natural gas, syngas from coal or biomass gasification, etc.) and the low NOx generation, since nitrogen is only introduced by fuel bound nitrogen or as a residue in the oxygen to the combustion chamber.

The generated NOx as well as other gases are removed together with CO2, so that no pollutants are emitted to atmosphere. This produces exhaust gas that is mainly water vapor and CO2. The exhaust gas has a relatively high CO2 concentration (greater than 80percent by volume).

Oxyfuel combustion represents an opportunity to improve the economics of CO2 capture.

2 Solvents and Sorbents for CO2 separation from flue gas (both physical and chemical) can be further enhanced to reduce cost, improve reaction rates and regeneration loads, and eliminate contamination from other pollutants. This includes technologies such as aqueous ammonia, advanced amines, ionic liquids, metal organic frameworks, and amine-enriched sorbents.

3 Advanced Membranes for both oxygen-separation and CO2 capture are key enabling technologies. This effort will evaluate needs for advanced membranes applicable to pulverized coal systems and other conventional combustion systems that will minimize the cost and efficiency losses for CO2 separation.

4 Chemical Looping processes that prevent direct contact of air and fuel offer the ability to produce a relatively pure stream of CO2 that does not need to be separated from flue gas. Technical challenges remain in key areas such as solids handling and oxygen carrier capacity, reactivity, and attrition.

Economic studies indicate that carbon capture will add over 30 percent to the cost of electricity for new integrated gasification combined cycle (IGCC) units and over 80 percent to the cost of electricity if retrofitted to existing pulverized coal (PC) units.

CO2 storage and use

Once captured and transported, most CO2 will be stored in geological reservoirs. The most of the countries are interested in a number of such reservoirs, including depleted and disused oil and gas fields, deep saline aquifers and deep un-mined coal seams or in the form of mineral carbonates.

Figure 2 - CO2 storage

CO2 Storage Options

Depleted Oil and Gas Fields

These present a significant possibility for CO2 storage, with European capacities estimated at 14.5 billion tonnes offshore and 13.1 billion tonnes .

Enhanced Oil (and Gas) Recovery

As an intermediate step, there is scope for injecting CO2 into mature fields to improve the recovery of oil (and gas) through Enhanced Oil Recovery (EOR), increasing production by 4-20%.

Saline aquifers

These have by far the greatest potential for storing CO2, globally as well as in Europe. Such aquifers are sedimentary rocks (usually sandstone and less frequently limestone or other rocks), which are porous enough to store great volumes of CO2 and permeable enough to allow the flow of fluids.

Storage of CO2 will take place at depths below some 7-800 meters where CO2 behaves as a fluid, and where the pores of the sediments are filled with salt water.

Un-mineable coal seams

These offer another opportunity to store CO2 at a low net cost. In Enhanced Coal Bed Methane (ECBM) projects if a production well is opened, the coal adsorbs CO2 and N2 and methane is displaced, enhancing its production. While this approach is still in its early stages and needs more research, it is considered a promising concept due to the added value of the produced methane.

Detailed knowledge and understanding are needed as to where and how CO2 can be stored. This understanding must include, for example, geographical locations, capacities, future behaviour in reservoirs, and associated risks, together with both national and international legal constraints. Attention must be given to the development of a monitoring methodology capable of building trust and confidence amongst citizens living in the vicinity of storage sites.

SOFC-GT

An SOFC-GT system is one which comprises a solid oxide fuel cell combined with a gas turbine. Further combination of the SOFC-GT in a combined heat and power plant also has the potential to yield even higher thermal efficiencies in some cases. In these plant SOFC is using as a replacement to combustor near gas turbine. It will generate electrical power at greater than 45% electrical efficiency.

Figure 3 - SOFC-GT

Within the SOFC module the desulfurized fuel is utilized electrochemically and oxidized below the temperature for NOx generation. Therefore NOx and SOx emissions for the SOFC power generation system are near negligible. The byproducts of the power generation from hydrocarbon fuels that are released into the environment are CO2 and water vapor.

The development of methods to capture and sequester the CO2, resulting in a Zero Emission power generation system.

Merits

1. These are compact, lower cost equipment
2. It has greater cycle efficiencies with advanced turbines.
3. Complete carbon capture results the Zero emission(ultra low emission)
4. The required amount of thermal energy is been supplied for desalination of water.
5. Zero Emission Power Plant has “triple benefit”.
• Carbon dioxide is captured and used for another application.
• Zero Emission power generated.
• Additional oil is produced from existing wells.

Demerits

1. The largest disadvantage is the high operating temperature which results in longer start-up times and mechanical and chemical compatibility issues.
2. Fuelling fuel cells is still a problem since the production, transportation, distribution and storage of hydrogen is difficult.
3. The cost of CO2 captures using current technology, of carbon – much too high for carbon emissions reduction applications.

Conclusion

In present scenario, combustion of fuels is at the heart of today’s power generating system. It leads to the emission of green house gases which causes Global warming. Among these gases CO2 constitutes the major proportion.

The development of methods to capture and sequester the CO2, results in a Zero Emission power generation system and hence overall performance and efficiency increases.

Voltage Transformer (VT) - Introduction And Purpose (pic by thomasnet.com)

Instrument transformers are primarily used to provide isolation between the main primary circuit and the secondary control and measuring devices. This isolation is achieved by magnetically coupling the two circuits. In addition to isolation, levels in magnitude are reduced to safer levels.

Instrument transformers are divided in to two categories: voltage transformers (VT) and current transformers (CT). The primary winding of the VT is connected in parallel with the monitored circuit, while the primary winding of the CT is connected in series.

The secondary windings proportionally transform the primary levels to typical values of 120 V and 5 A. Monitoring devices such as wattmeters, power-factor meters, voltmeters, ammeters, and relays are often connected to the secondary circuits.

Voltage transformer (VT)

The voltage transformer (VT) is connected in parallel with the circuit to be monitored. It operates under the same principles as power transformers, the significant differences being power capability, size, operating flux levels, and compensation. VTs are not typically used to supply raw power; however, they do have limited power ratings.

They can often be used to supply temporary 120-V service for light-duty maintenance purposes where supply voltage normally would not other wise be available. In switchgear compartments, they may beused to drive motors that open and close circuit breakers.

In voltage regulators, they may power a tap-changing drive motor. The power ranges are from 500 VA and less for low-voltage VT, 1–3 kVA for medium-voltage VT, and 3–5 kVA for high-voltage VT. Since they have such low power ratings, their physical size is much smaller. The performance characteristics of the VT are based on standard burdens and power factors, which are not always the same as the actual connected burden.

It is possible to predict , graphically, the anticipated performance when given at least two reference points. Manufacturers typically provide this data with each VT produced. From that, one can construct what is often referred to as the VT circle diagram, or fan curve, shown in Figure 1.

Knowing the ratio-error and phase-error coordinates, and the values of standard burdens, the graph can be produced to scale in terms of VA and power factor. Other power-factor lines can be inserted to pinpoint actual circuit conditions.

Figure 1 - Voltage transformer circle diagram (fan curves)

Performance can also be calculated using the same phasor concept by the following relationships, provided that the value of the unknown burden is less than the known burden. Two coordinates must be known: at zero and at one other standard burden value.

It's our sun forever, fossil fuels aren't and you know it.

Infinite energy resource

Solar energy is not a finite resource as fossil fuels are. While the sun is up there it constantly produces all the energy we can use.

Reduced maintenance costs

While not maintenance free, what technology really is? Once solar panels, wind or water power facilities are in place, no fuel or lubricants need to be supplied.

Falling production costs

The financial costs of producing appliances such as solar cells and solar hot water panels are falling as technology develops. Comparatively solar energy is competing with fossil fuels as fossil fuel prices have risen steeply globally in the last few years. Solar energy technology is becoming increasingly efficient.

Greatly reduced pollution

Why not pollution-free? Because it isn’t always. While having much better credentials than fossil fuel for polluting emissions, the environmental costs of manufacturing and constructing solar energy appliances must not be forgotten.

Also, consider the wider impacts of burning biomass and of large hydropower schemes. So, advantages of solar energy are still shadowed by some disadvantages. That’s just the necessary paradox of life.

Greatly reduced contribution to global warming

One of the greatest advantages of solar energy of course is that there are no carbon dioxide, methane or other emissions that warm the atmosphere. Again, manufacturing and installation of solar appliances are necessarily accompanied by some of those emissions.

Low running costs

With prices of traditional fuels soaring the cost advantages of solar energy are becoming obvious. After installation of the appliance, solar energy is free.

Local application

Suitable for remote areas that are not connected to energy grids. In some countries solar panels for domestic use in remote areas are becoming sources for local employment in manufacture and installation.

Fossil-fuel poor countries can kick their dependency on this energy and spend their funds on other things through application of solar energy.

Health and safety benefits

In some poorer countries where people have used kerosene and candles for domestic heating and lighting, respiratory diseases and impaired eyesight have resulted. Many people have been burned through accidents involving kerosene heating. Solar energy, especially with excess energy stored for night-time use, overcomes these problems.

Reliability

Among the significant advantages of solar energy is that of reliability. Local application and independence from a centrally controlled power grid and energy transport infrastructure is insurance from upheaval through political and economic turmoil.

It’s nice indeed when you can rely on your alternate energy sources to be able to prepare a hot meal for your family, turn on a light to read by, play a game or study, or operate your computer.

Solar energy grants

In some countries you will receive grants of money, rebates and tax reductions through implementing solar energy solutions in your home or business. It is also possible to “sell back” power into the grid and pay less for your electricity bill.

Remember my suggestion that the advantages of solar energy do not make a White Knight? I have already covered some of that with you but there are some further disadvantages of solar energy.

Some disadvantages of solar energy:

• It is available most abundantly in areas with a high number of sunshine hours. Where it is needed most, cold countries in high northern or southern latitudes, it is less easily captured and used.
• It is not directly available at night or under cloud cover and conversion into another form of energy storage systems are necessary for those times.
• DC power is produced by solar cells which must be converted to AC power before it can be used.

In the main these disadvantages are really engineering problems for which solutions are becoming increasingly available and efficient. They are not serious reasons to dismiss the value or utility of solar energy.

In summary, the advantages of solar energy are many. The drawbacks can be managed through making responsible use of solar technology and choosing those solar energy solutions that have a minimal impact on our environments.

Some commonly unacknowledged advantages of solar energy

Forms of highly localised solar energy reduce dependency on power monopolies and tend to benefit poor people. The associated advantages of solar energy, those of health and safety benefits, may reduce much suffering.

I cannot run away from it. I must face it and engage with it positively or perish in a heap of great frustration. Too often we still think we can run away from the effects of global warming. But our children will not be able to. Even David Attenborough, the eminent but hitherto sceptical Friend of the earth, is now adding his voice to the many warnings about global warming.

Using the right technology alone will not deliver a truly sustainable world where all are included to take part of its good things.

Using solar energy well, actually transforms us. Our creative efforts to use natural cycles and energy makes us more aware of our place in the world. Your increased awareness and mine leads us to be more responsible to one another and towards our environments.

In this way, using renewable energy involves you and me developing greater abilities to respond to each other kindly. This is perhaps among the greatest of advantages of solar energy.

What do you think?

Differential relay for transformer applications. Protection features include over-excitation (V/Hz), under/over-voltage, phase overcurrent, negative-sequence overcurrent, and breaker fail-to-trip/fail-to-close indicators. (pic by thomasnet.com)

1. High speed biased differential relay

The DMH type relay provides high speed biased differential protection for two or three winding transformers. The relay is immune to high inrush current and has a high degree of stability against through faults. It requires a max of two cycles operating time for current above twice relay rated current. Instantaneous overcurrent protection clears heavy internal faults immediately. This relay is available in two forms.

Firstly for use with time Cts, the ratios of line which are matched to the load current to give zero differential current under normal working conditions. Secondly with tapped interposing transformers for use with standard line current transformers of any ratio.

2. Directional inverse time overcurrent and earth fault relays

The CDD type relays are applied for directional or earth fault protection of ring mains, parallel transformers or parallel feeders with the time graded principle. It is induction disc type relay with induction cup used to add directional feature.

3. Instantaneous voltage relay

The type VAG relay is an instantaneous protection against abnormal voltage conditions such as over voltage, under voltage or no voltage in AC and DC circuits and for definite time operation when used with a timer. It is an attracted armature type relay.

4. Auxiliary relays

The VAA/CAA type auxiliary relays are applied for control alarm, indication and other auxiliary duties in AC or DC systems. CAA is a current operated and VAA is a voltage operated relay.. it is attracted armature type.

 5. High speed tripping relays

This VAJH type relay is employed with a high speed tripping duties where a number of simultaneous switching operations are required. This is a fast operating multi contact attracted armature relay.

6. Definite time delay relay

This VAT type relay is used in auto reclosing and control schemes and to provide a definite time feature for instantaneous protective relay. It is an Electro mechanical definite time relay. It has two pair of contacts. The shorter time setting is provided by a passing contact and longer time setting by the final contact.

7. Trip circuit supervision relay

This VAX relay is applied for after closing or continuous supervision of the trip circuit of circuit breakers.

They detect the following conditions:

1. Failure of trip relay
2. Open circuit of trip coil
3. Failure of mechanism to complete the tripping operation

8. Instantaneous over current and earth fault relay

An instantaneous phase or earth fault protection and for definite time operation when used with a timer. It is a CAG 12/12G standard attracted armature relay with adjustable settings. It may be a single pole or triple pole relay.

9. Inverse time over current and earth fault relay

This CDG 11-type relay is applied for selective phase and earth fault protection in time graded systems for AC machines. Transformers, feeders etc. this is a non-directional relay with a definite minimum time which has an adjustable inverse time/current characteristics. It may be a single pole or triple pole relay.

10.  Fuse failure relay

This VAP type relay is used to detect the failure or inadvertent removal of voltage transformer sec. fuses and to prevent incorrect tripping of circuit breaker. It is three units, instantaneous attracted armature type relay the coil of each unit connected across one of the VTs.

The secondary fuses under healthy conditions, the coil is SC by fuses and can’t be energized. But one or more fuses blow the coil is energized and relay operates.

11. Instantaneous high stability circulating current relay

It is used to serve the following three purposes

1. Differential protection of Ac machines , reactors auto transformers and bus bars
2. Balanced and restricted earth fault protection of generator of generator and transformer windings
3. Transverse differential protection of generators and parallel feeders.

This CAG type relay is a standard attracted armature relay. In circulating current protection schemes, the sudden and often asymmetrical growth of the system current during external fault conditions can cause the protection current transformers to go into saturation, resulting in high unbalance current to insure stability under these conditions.

The modern practice is to use a voltage operated high impedance relay, set to operate at a voltage slightly higher than that developed by CT under max fault conditions. Hence this type of relay is used with a stabilizing resistor.

12.   Local breaker back up relay

this is a CTIG type three phase or two phase earth fault instantaneous over current unit intended for use with a time delay unit to give back up protection in the event of a circuit breaker failure.

13. Poly-phase directional relay

The PGD relay is a high speed induction cup unit used to give directional properties to three phase IDMT over-current relays, for the protection of parallel feeders, inter connected networks and parallel transformers against phase to phase and three phase faults. Owing to low sensitivity on phase to earth faults the relay is used with discretion on solidly earthed systems.

14.  Auto reclose relay

Five types of auto reclose relays are available:

a) VAR21 giving one reclosure. The dead time and reclaim time are adjustable form 5 to 25 secs. If the circuit breaker reopens during reclaim time, it remains open and locked out.

b) VAR41B is a single shot scheme for air blast circuit breakers. Reclaim time is fixed at between 15 to 20 secs. Dead time adjustment is from 0.1 to 1.0 sec of which first 300 millisec will be circuit breaker opening time.

c) VAR 42 giving four reclosure. It is precision timed from 0 to 60 sec. it can be set for max four enclosures at min intervals of 10 sec and instantaneous protection can be suppressed after the first reclosure so that persistent faults are referred to time graded protection.

d) VAR 71 giving single shot medium speed reclosure with alarm and lockout for circuit breaker. This allows up to 10 faults clearance before initiating an alarm. The alarm is followed by lockout if selected no. of faults clearances exceed. If the circuit breaker reopens during reclaim time, it remains open and locked out. It offers delay in reclosing sequence. Instantaneous lockout on low current earth fault and suppressing instantaneous protection during reclamation time.

e) Var81 is a single shot high-speed reclosure with alarm and lockout for circuit breaker This allows up to 10 faults clearance before initiating an alarm.

Reactance distance scheme

this scheme consists of the following relays, XCG22-3 for phase to phase and 3 for phase to ground, YCG17, mho starting unit one in each place, VAT51 along with timing unit for zone 2 and 3, 86-X aux. tripping relay and 30G, H, and J for 1^st, 2^nd and 3^rd. Zone indication VAA51, CAG12 and VAA31. These schemes provide three zone phase and earth fault protection using reactance relays type XCG22 and also starting relays YCG17.

They are applicable to important line sections where high values of arc resistance would otherwise affect the accuracy of measurement and where high speed tripping is essential. High-speed protection is provided for phase and earth faults on 80-90% of the line section and faults on the remaining section are cleared in second zone, time. The third zone provides backup protection after further time interval.

Each mho starting unit Y3 and its auxiliary Y3 X is associated with one phase and operates for all faults involving this phase. Each reactance unit X is connected to measure phase or earth fault distance, but is prevented from operating by short circuit across the polarizing coils. Under the phase fault conditions, the Y3 X units unblock the appropriate X1 reactance units, which initiate tripping immediately for faults within their setting.

Operation of the earth auxiliary relay 64 in conjunction with the Y3 X units selects the appropriate reactance units for measurement of earth faults. The reach of reactance units is extended by the timer, 2 after successive intervals to cover faults in zone 2 and 3.

The principle of distance scheme - Block diagram

Discrimination is not affected by changing faults, for example a zone 2 earth fault which develops into a double phase to earth fault will be cleared correctly by the X1 (phase fault) units in zone 2 time. In the rare event of two faults occurring simultaneously at different points on the line; the scheme will measure to a distance approximately half way.

UPS Battery & Critical Reserve Power (photo by Infinity Power Solutions)

The 220V DC system supplies direct current as source of operating power for control, signaling, relays, tripping and closing of switchgears, emergency motors of most important auxiliary systems. Under normal conditions of station generation, the storage battery units are kept floating in DC bus bars by means of the trickle chargers (also known as float chargers). The trickle chargers of each battery unit, which is a rectifier with AC input, is normally made to take all DC requirements of the power station without allowing the battery to discharge. This is achieved by maintaining the DC output voltage of trickle charger a few volts higher than the voltage of the battery.

With this, the trickle charger besides meeting all the DC requirements of the power station, supplies a few hundred milliamps of direct current to the battery to compensate the loss in the capacity of the battery due to action between the plates of the cell. With this arrangement, the battery remains connected to the DC bus bars as a standby supply source and immediately supplies the DC load in the vent of temporary failure of complete AC system.

The complete AC power system failure in a power station is known as emergency situation. DC battery units are designed to supply station DC loads for an emergency period of one hour. The tickle charger normally supplies the station DC load and the momentary loads will also be catered for by the trickle charger and if such a load is more than its capacity, the battery being in parallel with the trickle charger will supply the excessive load. The trickle charger will normally be kept operating at around 115×2.15 V ie 247 volts. In case of AC mains failure the full battery of 115 cells will supply the load ie 230 volts. If the emergency lasts for one hour with an appropriate load of 450 Amps, then battery will supply the load for one hour when its end voltage will drop down to 1.75 volts per cell ie 201 volts.

After the emergency when the quick charger is closed the full battery will receive a boost charge and at the same time only the voltage of 98 cells will appear across the load.

If a second emergency occurs during quick charging, then immediately all the 115 cells are connected to the bus by closing the switch meant for the purpose. During routine daily testing of emergency DC motors connected to main distribution board middle section, supply has to be taken from the quick charger and the middle section has to be kept isolated from the left and right sections of main distribution board. This is to test the quick charger.

Procedure followed in commissioning a battery

1. The battery is charged initially to its capacity. The lead acid Battery has a capacity of 1000AH ie it may be charged for 10 hrs with charging current of 100 A or 5 hrs with charging current of 200 A. in case of Ni-Cd battery with a capacity of 2500 AH is charged for 12.5 hrs. with a charging current of 200A.
2. Now the battery is discharged at the rate of 10% of its capacity in case of lead-acid battery and 20% or 40% of its capacity in case of Ni-Cd battery.
3. Now the battery is recharged to its capacity.
4. Constant voltage charging of battery is called float charging. A lead acid battery of cell voltage 2.2V is float charged upto 2.42 V. A Ni-Cd battery of cell voltage 1.2V is float charged upto 1.41 V.
5. Constant current charging of a battery is called boost charging. A lead acid battery with bank voltage 237 may be boost charged to 279V. A Ni-Cd battery with bank voltage 242 may be boost charged to 283V.

Equipment used in 220V DC supply system

Sources of AC power

Two sources of AC power have been provided for both quick charger and trickle charger, one is the normal source and other is standby. AC power supply to the chargers is through transformers having off-load tap changing arrangement. An AC voltage-signaling relay communicates; ‘AC voltage low’ when the supply voltage becomes low.

Voltage level indicating device

A voltage level indicating device in MDB gives audio and visual annunciation when the DC bus voltage changes beyond set low (180-210) and high limits (240-270).

AVR

The DC voltage is maintained at desired value automatically by means of AVR unit provided at panel board.

Insulation monitoring device

This device annunciates when the insulation resistance of either positive bus to earth or negative bus to earth falls below 20 kilo ohms and also when the ratio of insulation resistance of positive bus to earth to negative bus to earth is 1.5 or above.

Flickering light device

This has been installed in the MDB, for flicker supply to control and check whether device is in order or not. Control and signaling panels have two sets of bus bars, one fed by main distribution board left section and the other by MDB right section. The loads of the first panel should be kept switched to the set of bus bars fed by MDB. Left section and the loads of the second panel should be kept connected to the set of bus bars fed by MDB right section.

Electrostatic Precipitator

Dust extraction from industrial gases has become necessity for environmental reasons or for improving production. Most of the plants in India use coal as fuel for generating steam. The exhaust gases contain large amount of smoke and dust, which are being emitted into the atmosphere. This has posed a real threat to the mankind as a devastating health hazard. Hence it becomes necessary to free the exhaust from smoke and dust.

There are various ways of extracting dust. Electrostatic dust precipitation method is most widely used as its efficiency is excellent and it is easier to maintain. Its other advantages are:

• Ability to treat large volumes of gases at high temperature
• Ability to cope up corrosive atmosphere
• Offer low resistance path for gas flow.

An electrostatic precipitator is equipment, which utilizes an intense electric force to separate suspended particles from the flue gases. The process involves:

• Electrical charging of suspended particles
• Collection of charged particles from collecting electrode.
• Removal of particles from collecting electrode.

The flue gases pass between electrodes and are subjected to an intense electric field. The emissive electrodes are connected to the negative polarity of HV power supply while collecting electrodes are connected to positive polarity and grounded.

Heaters

Heaters are provided to raise the temperature of flue gases, as they become conductive when heated. 24 heaters are provided for stage I electrostatic precipitators. Rating: 550W heaters

Zones

The flue gases from the boiler section reach electrostatic precipitator section through ducts. The flue gases are allowed to pass through various zones each having its own heaters, collecting and emissive electrodes and DC supply. These zones are provided to lessen the burden on a single zone and to take the load of other zone in case of maintenance or damage of a particular zone. Stage I have 16 zones eight belonging to PASS A and rest to PASS B. Stage II has 20 Zones five belonging to each PASS A, PASS B, PASS C and PASS D.

Diodes

These are provided to rectify the AC voltage to the required DC voltage for electrostatic precipitators to work. The required DC voltage is 70 kV, 1000 mA. Type: BY 127

Motors

Rapping motors are provided along with each zone. A hammer is coupled to each of the motor’s shaft. Due to rotary motion of motor these hammers hit the collecting electrodes after a certain time delay and the ash is allowed to flow down through outlet in form of slurry. Rating: .5A motors

A GD screen (gas diverting) motor is also provided in electrostatic precipitator to provide a zigzag motion of flue gas so as to allow the heavy dust particles to settle down and removed.

Features:

Spark regulation

Flashovers of extremely low intensity are difficult to detect using the comparator technique. Non detection results in sustained arcing which may damage the collecting electrode. For such digital detection system is adopted.

Fast ramp control

In case of fast changes in operating conditions of precipitator many sparks may occur within a short time reducing current to a low value, when the disturbance disappears, it may take a relatively long time before the current can assume its normal value. This is the case particularly if selected rate of rise is low.

Modes of operation

Back corona mode

In this mode the precipitator voltage decreases with increase in precipitator current. This reduces the efficiency of precipitator and consumes unnecessary power.

Charge ratio mode

In a high resistive dust a potential gradient is created within the dust layers which causes occurrence of local sparks in dust layer. This spurious discharges or BACK CORONA occurs as soon as potential gradient is high. This has negative impact on efficiency.

Charge ratio

This mode supplies current in pulses and provides a dense corona for a short circuit time and at same time gives a low current to avoid back corona.

Solar panels powering a rural cultural and drama centre, near Auroreville, Pondicherri

“Solar energy is free, but it’s not cheap” best sums up the major hurdle for the solar industry. There are no technical obstacles per se to developing solar energy systems, even at the utility megaWatt level (e.g., 14 MW utility scale PV system at Nellis AFB or a 64-MW CSP system in Nevada); however, at such large scales a high initial capital investment is required.

Over the past three decades, a significant reduction of the cost of solar products has occurred, without including environmental benefits; yet, solar power is still considered a relatively expensive technology. For small- and medium-scale uses, in some applications, such as passive solar design for homes, the initial cost of a home designed to use solar power is essentially no more than that of a regular home, and operating costs are much less.

The only difference is that the solar-energy home works with the Sun throughout the year and needs smaller mechanical systems for cooling and heating, while poorly designed homes fight the Sun and are iceboxes in the winter and ovens in the summer.

As the full effect and impact of environmental externalities such as global warming become apparent, society will demand cleaner energy technologies and policies that favor development of a clean-energy industrial base. By the end of the twenty-first century, clean-energy sources will dominate the landscape.

This will not be an easy or cheap transition for society, but it is necessary and inevitable.

Rural Systems

Rural Pakistani village

Already, solar energy is cost effective for many urban and rural applications. Solar hot-water systems are very competitive, with typical paybacks from 5–7 years as compared to electric hot-water heaters (depending on the local solar resource).

PV systems are already cost competitive for sites that are remote from the electric grid, although they are also popular for on-grid applications as environmental “elitists” try to demonstrate that they are “green.

”However, one should beware of “green-washing” as people and companies install grid-tied PV systems without making efforts to install energy-efficient equipment first. Far more can be achieved through energy conservation than solar energy usage alone for reducing carbon emissions.

The decision to use a solar energy system over conventional technologies depends on the economic, energy security, and environmental benefits expected. Solar energy systems have a relatively high initial cost; however, they do not require fuel and often require little maintenance. Due to these characteristics, the long-term life cycle costs of a solar energy system should be understood to determine whether such a system is economically viable.

Historically, traditional business entities have always couched their concerns in terms of economics. They often claim that a clean environment is uneconomical or that renewable energy is too expensive. They want to continue their operations as in the past because, sometimes, they fear that if they have to install new equipment, they cannot compete in the global market and will have to reduce employment, jobs will go overseas, rates must increase, etc.

The different types of economics to consider are pecuniary, social, and physical. Pecuniary is what everybody thinks of as economics: dollars. Social economics are those borne by everybody and many businesses want the general public to pay for their environmental costs. If environmental problems affect human health today or in the future, who pays? Physical economics is the energy cost and the efficiency of the process. There are fundamental limitations in nature due to physical laws. In the end, the environment and future generations always suffer the corollary of paying now or probably paying more in the future.

An economical analysis should be looking at life cycle costs, rather than at just the ordinary way of doing business and low initial costs. Life cycle costs refer to all costs over the lifetime of the system. Also, incentives and penalties for the energy entities should be accounted for.

What each entity wants is to earn subsidies for itself and penalties for its competitors. Penalties come in the form of taxes and fines; incentives may come in the form of tax breaks, unaccounted social and environmental costs, and also what the government (society) could pay for research and development.

Protection of generators and transformers (Siprotec numerical protection relays)

As Generators and transformers are major components of a power system, so it is quite necessary to take all the preventive measures for the protection of transformers and generators.

These are the following ways (ANSI codes) we use to protect transformers and generators from faults:

87/G1

Generator differential protection (87/G1): the protection is provided with high speed, high stability circulating current relays. The relays has a pick up range of 10 to 40% of 5A and shall have suitable stabilizing for ensuring stability against external faults. The relays shall be tuned to fundamental frequency to reject harmonics produced by CT saturation.

87/GT1

Generator-transformer differential protection (87/GT1): the diff. relays for the generator-transformer has to be of sensitive high speed percentage bias type with harmonic restraint and has CT ratio matching devices.

The relay should have all required restraints to make it inoperative for fault current, magnetizing inrush current and abnormal magnetizing inrush current during short time over voltage conditions.

87T1A

Unit auxiliary transformer differential protection (87T1A): percentage biased differential relays are used along with ratio matching device. The relay shall not operate for magnetizing inrush current. High set instantaneous over current relays shall be provided in series with the previous relay, set for magnetizing inrush current, for fast protection from internal faults.

64GI

Generator stator earth fault protection (64GI): the stator earth fault protection shall consist of a zero sequence voltage relay connected to the broken delta winding of generator voltage transformer. The protection shall act to initiate a time-delayed signal only and hence the relay shall be continuously rated for 110V.

The relay shall incorporate arrangements to make it insensitive to third harmonic voltages.

87TG

Generator inter-turn protection(87TG): the protection shall be by means of an instantaneous over current relay. The relay shall incorporate filtering arrangements to make it inoperative for third harmonics. The relay shall have suitable range to cover 20-50% of generator current.

64-1,64-2

Generator rotor earth fault protection(64-1,64-2): the first rotor earth fault of generator shall be detected by means of super imposing of separate Dc bias on the field winding. The DC bias shall be such that the faults at any point in the winding are covered by the protection. Also the relay shall withstand the voltage encountered. Second rotor earth fault protection for generators shall also be provided, with suitable relays common for two units.

The protection shall incorporate feature for compensating the effects of induced alternating currents in rotor circuit and shall have minimum dead zone.

40G

Generator loss of field protection(40G): this protection shall be single phase off set impedance type. The relays shall have impedance settings to cover the usual range of impedance of large turbo generators.

21G

Generator backup protection(21G): the generator backup relay shall be of three phase impedance type for one zone protection together with required auxiliary relays and two stage timer to give backup protection for faults in the generator, main transformer and transmission system.

46G

Generator negative sequence current protection(46G): the negative sequence current relays shall protect the generator from damage by overheating due to sustained flow of unbalanced phase currents, and the operating characteristics of the relay shall be adjustable to match I2Rt thermal characteristics

67-1G, 67-2G

Generator under power and anti motoring protection (67-1G, 67-2G): the step up transformer back up earth fault protection current relays shall be of IDMT characteristics. One of the two relays shall be set with higher time dial setting to provide second stage of protection.

51G

Generator overload protection(51G): one over current relay shall be provided to initiate an overload alarm. The relay shall have high reset ratio and adequate continuous thermal rating.

59G

Generator over voltage protection(59G): suitable over-voltage relays preferably with volt/cycle characteristics shall be provided.

50T1A

Back-up protection for unit auxiliary transformer(50T1A): Two instantaneous over current relays with an external DC timer shall be provided for back-up protection for unit 6.6 kV bus bar.

95G

Fuse failure protection(95G): This protection shall block the operation of all protections associated with voltage circuits in the event of failure of a fuse of generator.

86G and 86GT

Lockout relays(86G and 86GT): lockout relays shall be provided for each generator unit which will be multi contact , hand reset type. The latching mechanism shall be positive and insensitive to vibration and shock

Low voltage switchgear - Power distribution (by MEC Electrical Engineering)

Continued from Types of neutral earthing in power distribution (part 1)

3. Resistance earthed systems

Resistance grounding has been used in three-phase industrial applications for many years and it resolves many of the problems associated with solidly grounded and ungrounded systems. Resistance Grounding Systems limits the phase-to-ground fault currents.

Grounding Resistors are generally connected between ground and neutral of transformers, generators and grounding transformers to limit maximum fault current as per Ohms Law to a value which will not damage the equipment in the power system and allow sufficient flow of fault current to detect and operate Earth protective relays to clear the fault. Although it is possible to limit fault currents with high resistance Neutral grounding Resistors, earth short circuit currents can be extremely reduced.

As a result of this fact, protection devices may not sense the fault.

Therefore, it is the most common application to limit single phase fault currents with low resistance Neutral Grounding Resistors to approximately rated current of transformer and / or generator.

In addition, limiting fault currents to predetermined maximum values permits the designer to selectively coordinate the operation of protective devices, which minimizes system disruption and allows for quick location of the fault.

There are two categories of resistance grounding:

1. Low resistance Grounding
2. High resistance Grounding

Ground fault current flowing through either type of resistor when a single phase faults to ground will increase the phase-to-ground voltage of the remaining two phases. As a result, conductor insulation and surge arrestor ratings must be based on line-to-line voltage. This temporary increase in phase-to-ground voltage should also be considered when selecting two and three pole breakers installed on resistance grounded low voltage systems.

The increase in phase-to-ground voltage associated with ground fault currents also precludes the connection of line-to-neutral loads directly to the system. If line-to neutral loads (such as 277V lighting) are present, they must be served by a solidly grounded system. This can be achieved with an isolation transformer that has a three-phase delta primary and a three-phase, four-wire, wye secondary.

Resistor neutral earthing

Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated with phase-to-phase faults, but both systems significantly reduce or essentially eliminate the arc-flash hazards associated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses and reduce thermal damage to electrical equipment, circuits, and apparatus carrying faulted current.

The difference between Low Resistance Grounding and High Resistance Grounding is a matter of perception and, therefore, is not well defined. Generally speaking high-resistance grounding refers to a system in which the NGR let-through current is less than 50 to 100 A.Low resistance grounding indicates that NGR current would be above 100 A.

A better distinction between the two levels might be alarm only and tripping. An alarm-only system continues to operate with a single ground fault on the system for an unspecified amount of time. In a tripping system a ground fault is automatically removed by protective relaying and circuit interrupting devices. Alarm-only systems usually limit NGR current to 10 A or less.

Rating of The Neutral grounding resistor:

1. Voltage: Line-to-neutral voltage of the system to which it is connected.
2. Initial Current: The initial current which will flow through the resistor with rated voltage applied.
3. Time: The “on time” for which the resistor can operate without exceeding the allowable temperature rise.

A. Low Resistance Grounded

Low Resistance Grounding is used for large electrical systems where there is a high investment in capital equipment or prolonged loss of service of equipment has a significant economic impact and it is not commonly used in low voltage systems because the limited ground fault current is too low to reliably operate breaker trip units or fuses. This makes system selectivity hard to achieve. Moreover, low resistance grounded systems are not suitable for 4-wire loads and hence have not been used in commercial market applications.

A resistor is connected from the system neutral point to ground and generally sized to permit only 200A to 1200 amps of ground fault current to flow. Enough current must flow such that protective devices can detect the faulted circuit and trip it off-line but not so much current as to create major damage at the fault point.

Low resistance grounded

Since the grounding impedance is in the form of resistance, any transient over voltages are quickly damped out and the whole transient overvoltage phenomena is no longer applicable. Although theoretically possible to be applied in low voltage systems (e.g. 480V),significant amount of the system voltage dropped across the grounding resistor, there is not enough voltage across the arc forcing current to flow, for the fault to be reliably detected.

For this reason low resistance grounding is not used for low voltage systems (under 1000 volts line to-line).

Advantages

1. Limits phase-to-ground currents to 200-400A.
2. Reduces arcing current and, to some extent, limits arc-flash hazards associated with phase-to-ground arcing current conditions only.
3. May limit the mechanical damage and thermal damage to shorted transformer and rotating machinery windings.

Disadvantages:

1. Does not prevent operation of over current devices.
2. Does not require a ground fault detection system.
3. May be utilized on medium or high voltage systems.
4. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer.
5. Used: Up to 400 amps for 10 sec are commonly found on medium voltage systems.

B. High Resistance Grounded

High resistance grounding is almost identical to low resistance grounding except that the ground fault current magnitude is typically limited to 10 amperes or less. High resistance grounding accomplishes two things.

The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable damage is done at the fault point. This means that the faulted circuit need not be tripped off-line when the fault first occurs. Means that once a fault does occur, we do not know where the fault is located. In this respect, it performs just like an ungrounded system.

The second point is it can control the transient overvoltage phenomenon present on ungrounded systems if engineered properly.

Under earth fault conditions, the resistance must dominate over the system charging capacitance but not to the point of permitting excessive current to flow and thereby excluding continuous operation.

High resistance grounded

High Resistance Grounding (HRG) systems limit the fault current when one phase of the system shorts or arcs to ground, but at lower levels than low resistance systems.

In the event that a ground fault condition exists, the HRG typically limits the current to 5-10A.

HRG’s are continuous current rated, so the description of a particular unit does not include a time rating. Unlike NGR’s, ground fault current flowing through a HRG is usually not of significant magnitude to result in the operation of an over current device. Since the ground fault current is not interrupted, a ground fault detection system must be installed.

These systems include a bypass contactor tapped across a portion of the resistor that pulses (periodically opens and closes). When the contactor is open, ground fault current flows through the entire resistor. When the contactor is closed a portion of the resistor is bypassed resulting in slightly lower resistance and slightly higher ground fault current.

To avoid transient over-voltages, an HRG resistor must be sized so that the amount of ground fault current the unit will allow to flow exceeds the electrical system’s charging current. As a rule of thumb, charging current is estimated at 1A per 2000KVA of system capacity for low voltage systems and 2A per 2000KVA of system capacity at 4.16kV.

These estimated charging currents increase if surge suppressors are present. Each set of suppressors installed on a low voltage system results in approximately 0.5A of additional charging current and each set of suppressors installed on a 4.16kV system adds 1.5A of additional charging current.

A system with 3000KVA of capacity at 480 volts would have an estimated charging current of 1.5A.Add one set of surge suppressors and the total charging current increases by 0.5A to 2.0A. A standard 5A resistor could be used on this system. Most resistor manufacturers publish detailed estimation tables that can be used to more closely estimate an electrical system’s charging current.

Advantages

1. Enables high impedance fault detection in systems with weak capacitive connection to earth
2. Some phase-to-earth faults are self-cleared.
3. The neutral point resistance can be chosen to limit the possible over voltage transients to 2.5 times the fundamental frequency maximum voltage.
4. Limits phase-to-ground currents to 5-10A.
5. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase-to-ground arcing current conditions only.
6. Will eliminate the mechanical damage and may limit thermal damage to shorted transformer and rotating machinery windings.
7. Prevents operation of over current devices until the fault can be located (when only one phase faults to ground).
8. May be utilized on low voltage systems or medium voltage systems up to 5kV. IEEE Standard 141-1993 states that “high resistance grounding should be restricted to 5kV class or lower systems with charging currents of about 5.5A or less and should not be attempted on 15kV systems, unless proper grounding relaying is employed”.
9. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer.

Disadvantages

1. Generates extensive earth fault currents when combined with strong or moderate capacitive connection to earth Cost involved.
2. Requires a ground fault detection system to notify the facility engineer that a ground fault condition has occurred.

4. Resonant earthed system

Adding inductive reactance from the system neutral point to ground is an easy method of limiting the available ground fault from something near the maximum 3 phase short circuit capacity (thousands of amperes) to a relatively low value (200 to 800 amperes).

To limit the reactive part of the earth fault current in a power system a neutral point reactor can be connected between the transformer neutral and the station earthing system.

A system in which at least one of the neutrals is connected to earth through an

1. Inductive reactance.
2. Petersen coil / Arc Suppression Coil / Earth Fault Neutralizer.

The current generated by the reactance during an earth fault approximately compensates the capacitive component of the single phase earth fault current, is called a resonant earthed system.

The system is hardly ever exactly tuned, i.e. the reactive current does not exactly equal the capacitive earth fault current of the system.

A system in which the inductive current is slightly larger than the capacitive earth fault current is over compensated. A system in which the induced earth fault current is slightly smaller than the capacitive earth fault current is under compensated.

Resonant neutral earthing

However, experience indicated that this inductive reactance to ground resonates with the system shunt capacitance to ground under arcing ground fault conditions and creates very high transient over voltages on the system. To control the transient over voltages, the design must permit at least 60% of the 3 phase short circuit current to flow underground fault conditions.

Example – A 6000 amp grounding reactor for a system having 10,000 amps 3 phase short circuit capacity available. Due to the high magnitude of ground fault current required to control transient over voltages, inductance grounding is rarely used within industry.

Petersen Coils

A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that the capacitive current in the earth fault is compensated by an inductive current passed by the Petersen Coil. A small residual current will remain, but this is so small that any arc between the faulted phase and earth will not be maintained and the fault will extinguish. Minor earth faults such as a broken pin insulator, could be held on the system without the supply being interrupted. Transient faults would not result in supply interruptions.

Although the standard ‘Peterson coil’ does not compensate the entire earth fault current in a network due to the presence of resistive losses in the lines and coil, it is now possible to apply ‘residual current compensation’ by injecting an additional 180° out of phase current into the neutral via the Peterson coil. The fault current is thereby reduced to practically zero. Such systems are known as ‘Resonant earthing with residual compensation’, and can be considered as a special case of reactive earthing.

Resonant earthing can reduce EPR to a safe level. This is because the Petersen coil can often effectively act as a high impedance NER, which will substantially reduce any earth fault currents, and hence also any corresponding EPR hazards (e.g. touch voltages, step voltages and transferred voltages, including any EPR hazards impressed onto nearby telecommunication networks).

Advantages

1. Small reactive earth fault current independent of the phase to earth capacitance of the system.
2. Enables high impedance fault detection.

Disadvantages

1. Risk of extensive active earth fault losses.
2. High costs associated.

5. Earthing Transformers

For cases where there is no neutral point available for Neutral Earthing (e.g. for a delta winding), an earthing transformer may be used to provide a return path for single phase fault currents.

Earthing transformers

In such cases the impedance of the earthing transformer may be sufficient to act as effective earthing impedance. Additional impedance can be added in series if required. A special ‘zig-zag’ transformer is sometimes used for earthing delta windings to provide a low zero-sequence impedance and high positive and negative sequence impedance to fault currents.

Conclusion

Resistance Grounding Systems have many advantages over solidly grounded systems including arc-flash hazard reduction, limiting mechanical and thermal damage associated with faults, and controlling transient over voltages.

High resistance grounding systems may also be employed to maintain service continuity and assist with locating the source of a fault.

When designing a system with resistors, the design/consulting engineer must consider the specific requirements for conductor insulation ratings, surge arrestor ratings, breaker single-pole duty ratings, and method of serving phase-to-neutral loads.

Comparison of Neutral Earthing System

Reference:

• By Michael D. Seal, P.E., GE Senior Specification Engineer.
• IEEE Standard 141-1993, “Recommended Practice for Electrical Power Distribution for Industrial Plants”
• Don Selkirk, P.Eng, Saskatoon, Saskatchewan Canada

Figure 2 - In high-voltage cables the insulating material makes up a greater fraction of the total cross-sectional area than the conductor material

Space is really a critical criterion when discussing electrical cables and wires. In a low-voltage (LV) plastic-sheathed cable with conductor cross-sections of up to 10 mm2 per conductor or in high-voltage (HV) cables (Figure 2), the lion’s share of the cross-sectional area is occupied by the insulating material.

If aluminium rather than copper is used as the conductor material, the additional cross-sectional area required is more or less negligible in comparison.

Left: Figure 3 - Mineral-insulated cables; Right: Figure 4 - The structure of fireproof plastic-coated cable and mineral-insulated cable

At least that is the situation for conventional plastic-coated cables. Mineral-insulated cables and wires (Figure 3) are not only absolutely fireproof, they also take up much less space (Figure 4) than conventional plastic-sheathed cables.

For a time, these mineral-insulated cables were even equipped with an aluminium sheath, but this never became established and copper sheathing remains the norm.

And in most European countries, copper is still used predominantly, if not exclusively, for electrical installation work in buildings.

So why is it that most European standards do not permit the use of aluminium conductors with cross-sections up to 16 mm² (or in some cases) up to 10 mm²?

There are three main reasons:

1 Although aluminium is quite ductile, it is not as ductile as copper. The ends of stif wires laid in walls e.g. as connections to flush-mounted sockets or wall outlets tend to break after being repeatedly bent back and forth. This can be problematic if the imminent fracture point is located inside the insulating sheath and if the wire continues to be used. In such cases the fault can remain undetected until the wire has to carry a sizeable current (that is one close to its rated maximum current) and although it could be years before this situation arises, when it does, the conductor material will melt at the fracture point and sustained arcing can occur.

In the worst case, this can cause the aluminium to catch fire and burn like a fuse wire.

2 When exposed to air, the surface of aluminium rapidly becomes covered by a hard, durable oxide layer that does not conduct electricity, thus making it harder to ensure electrical contact. The build up of oxide at points where aluminium wires are terminated or connected, can increase the local electrical resistance of the conductor. The increased resistance can cause elevated temperatures with the risk of heat damage to the insulating materials and possibly fire.

Copper also undergoes oxidation when exposed to air, but perhaps surprisingly, the oxide layer does not inhibit electrical contact, even though the copper oxides (CuO and Cu₂O) have conductivities some 13 orders of magnitude less than elementary copper and can therefore hardly be described as electrical conductors.

3 Aluminium has a propensity to undergo slow material creep. When subjected to high pressures, the material will yield over time. One result of this is that originally tight connections may gradually become loose.

Connection technology is available that can deal with this problem and it is worth investing the extra cost and ef ort involved for installations involving relatively few connection points (e.g. HV overhead transmission lines), but not for more complex branched networks such as those found inside buildings.

Concluding…

Because of the second of the three problems listed above, connections involving the ends of aluminium conductors should always be made as tight screw-fastened contacts.

Unfortunately, the third problem discussed above means that these joints are often not permanent. Spring con tacts can be helpful, but they tend to suf er from the problems associated with the insulating aluminium oxide layers. In both cases, the result is a slow rise in the contact resistance at the connection point and thus to an increased risk of fire.

Fortunately, methods are now available for ensuring proper electrical contact between these older ‘protected‘ installations and newer electrical systems. These connectors combine spring-loaded contacts with a special contact paste made from grease and sharp metal particles.

When the connection is made the particles penetrate the existing aluminium oxide layer while the grease protects the contact area from renewed corrosion.

Figure 6 - Only in low-voltage high-current cables does the conductor material make up most of the cable's total cross-sectional area

Copper is also the preferred conductor material in high-voltage cables. Although the use of aluminium would result in only a slight increase in the overall conductor cross section, the insulating materials and the exterior shielding required for HV cables are expensive and the greater total cross-sectional area of the cable would cancel out the savings made by using the cheaper conductor material – in contrast to the situation with low-voltage power cables (Figure 6).

If aluminium is chosen as the conductor material, then processing the scrap cable at the end of its (admittedly long) service life will involve the additional step of separating the two materials.

About 45 % of the copper required today is generated from scrap, and the products for which it is used (cables, transformers, water pipes or rooi ng) will remain in use for a long time, on average around forty years. However, forty years ago, the demand for copper was only about half of what it is today.

It follows that about 90 % of the copper used at that time is still in use today. This applies equally to aluminium and other metals. Metals are not consumed, they are used.

Low voltage switchgear - Power distribution (by MEC Electrical Engineering)

Introduction

In the early power systems were mainly Neutral ungrounded due to the fact that the first ground fault did not require the tripping of the system. An unscheduled shutdown on the first ground fault was particularly undesirable for continuous process industries. These power systems required ground detection systems, but locating the fault often proved difficult. Although achieving the initial goal, the ungrounded system provided no control of transient over-voltages.

A capacitive coupling exists between the system conductors and ground in a typical distribution system. As a result, this series resonant L-C circuit can create over-voltages well in excess of line-to-line voltage when subjected to repetitive re-strikes of one phase to ground.

This in turn, reduces insulation life resulting in possible equipment failure.

Neutral grounding systems are similar to fuses in that they do nothing until something in the system goes wrong. Then, like fuses, they protect personnel and equipment from damage. Damage comes from two factors, how long the fault lasts and how large the fault current is. Ground relays trip breakers and limit how long a fault lasts and Neutral grounding resistors limit how large the fault current is.

Importance of Neutral Grounding

There are many neutral grounding options available for both Low and Medium voltage power systems. The neutral points of transformers, generators and rotating machinery to the earth ground network provides a reference point of zero volts.

This protective measure offers many advantages over an ungrounded system, like:

1. Reduced magnitude of transient over voltages
2. Simplified ground fault location
3. Improved system and equipment fault protection
4. Reduced maintenance time and expense
5. Greater safety for personnel
6. Improved lightning protection
7. Reduction in frequency of faults.

Methods of Neutral Earthing

There are five methods for Neutral earthing:

1. Unearthed Neutral System
2. Solid Neutral Earthed System
3. Resistance Neutral Earthing System
   • Low Resistance Earthing
   • High Resistance Earthing
4. Resonant Neutral Earthing System
5. Earthing Transformer Earthing

1. Ungrounded Neutral Systems

In ungrounded system there is no internal connection between the conductors and earth. However, as system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces. Consequently, the “ungrounded system” is, in reality, a “capacitive grounded system” by virtue of the distributed capacitance.

Under normal operating conditions, this distributed capacitance causes no problems. In fact, it is beneficial because it establishes, in effect, a neutral point for the system; As a result, the phase conductors are stressed at only line-to-neutral voltage above ground.

But problems can rise in ground fault conditions. A ground fault on one line results in full line-to-line voltage appearing throughout the system. Thus, a voltage 1.73 times the normal voltage is present on all insulation in the system.

This situation can often cause failures in older motors and transformers, due to insulation breakdown.

Ungrounded neutral system

Advantages

After the first ground fault, assuming it remains as a single fault, the circuit may continue in operation, permitting continued production until a convenient shut down for maintenance can be scheduled.

Disadvantages

1. The interaction between the faulted system and its distributed capacitance may cause transient over-voltages (several times normal) to appear from line to ground during normal switching of a circuit having a line-to ground fault (short). These over voltages may cause insulation failures at points other than the original fault.
2. A second fault on another phase may occur before the first fault can be cleared. This can result in very high line-to-line fault currents, equipment damage and disruption of both circuits.
3. The cost of equipment damage.
4. Complicate for locating fault(s), involving a tedious process of trial and error: first isolating the correct feeder, then the branch, and finally, the equipment at fault. The result is unnecessarily lengthy and expensive down downtime.

2. Solidly Neutral Grounded Systems

Solidly grounded systems are usually used in low voltage applications at 600 volts or less. In solidly grounded system, the neutral point is connected to earth.

Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of the system three phase fault current..

However, if the reactance of the generator or transformer is too great, the problem of transient over voltages will not be solved.

While solidly grounded systems are an improvement over ungrounded systems, and speed up the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides.

To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must be extend from the source to the furthest point of the system within the same raceway or conduit. Its purpose is to maintain very low impedance to ground faults so that a relatively high fault current will flow thus insuring that circuit breakers or fuses will clear the fault quickly and therefore minimize damage.

Solidly Neutral Grounded Systems

It also greatly reduces the shock hazard to personnel!

If the system is not solidly grounded, the neutral point of the system would “float” with respect to ground as a function of load subjecting the line-to-neutral loads to voltage unbalances and instability. The single-phase earth fault current in a solidly earthed system may exceed the three phase fault current. The magnitude of the current depends on the fault location and the fault resistance.

One way to reduce the earth fault current is to leave some of the transformer neutrals unearthed.

Advantages

The main advantage of solidly earthed systems is low over voltages, which makes the earthing design common at high voltage levels (HV).

Disadvantages

1. This system involves all the drawbacks and hazards of high earth fault current: maximum damage and disturbances.
2. There is no service continuity on the faulty feeder.
3. The danger for personnel is high during the fault since the touch voltages created are high.

Applications

1. Distributed neutral conductor
2. 3-phase + neutral distribution
3. Use of the neutral conductor as a protective conductor with systematic earthing at each transmission pole
4. Used when the short-circuit power of the source is low

References:

• By Michael D. Seal, P.E., GE Senior Specification Engineer.
• IEEE Standard 141-1993, “Recommended Practice for Electrical Power Distribution for Industrial Plants”
• Don Selkirk, P.Eng, Saskatoon, Saskatchewan Canada