Why are power engineers interested in Very High Voltages? Can't they do it in low voltage?
According to the Ohm's theorem;
Power loss in a conductor, P' = I^2 * R
=> So the power loss is directly proportional to the square of the line current.
=> Therefore to reduce losses, 'I' should be minimised.
But,
The Power requirement is fixed. Then by looking at the equation -
Apparent power, P = V * I
=> To minimise I;
=>V should be maximised.
That leads to the conclusion that transmission voltage should be increased to the MAXIMUM possible value.
Can we do that?
Even if we produce a three phase voltage of 1,000,000,000 V for argument's sake, can our equipments withstand it?
starting from the power cable of transformers, transformer terminals, power conductors, line towers, insulators, switchgear, and numerous things in a line SHOULD be made to withstand that same 1,000,000,000 V - Which is nearly impossible in present conditions.
So it boils down to simply an issue of trade-off.
=> bigger the voltage better; smaller the current better. for that it means a VERY big investment. So let us reduce some voltage. Then the current flow is going to increase. So as the line losses!
Three types of matter:
As matter is present in 3 kinds, so as the electrical insulation is of following types:
1. Solid
2. Liquid
3. Gas
HV breakdown:
High voltage naturally consists of an electric field pattern, and it depends on the voltage. It implies that certain insulation is capable of catering a certain maximum voltage. Beyond that, it will exhibit signs of leakage. That may sometimes lead to complete breakdown of that insulation medium. This value for still air is about 30 kV per cm.
HV insulators are of special design in order to create a longer creepage path. This avoids flashovers to some extent. But when dust particles and particulate matter get deposited on the insulator surface, and when rain water falls on that dirty surface, flashovers become frequent. Proper maintenanance of Hv insulators are thus essential for power quality.
Corona:
We can say corona is a phenomenon which occurs near a HV conductor. This effect may have been observed by many people residing under HV lines and especially near line supports.
An electrical field (stress) will be present around any electrical conductor. In HV conductors, the field strength would be higher. When this value increases beyond a certian limit, it starts to ionise the sorrounding air molecules. When sufficient ions are formed around that line, a portion of air becomes conductive (or called leakage). This is called 'corona effect'.
Corona has the following features, which are clearly visible in night time.
1. Hissing sound
2. Bluish/violet glow
This certainly means there is a power loss due to corona effect.
It should be noted that the electrical field strength will become more in bent/sharp conductors. Near the pole supports conductors need to bent for spur connections/ tensioning. That's why we observe corona near pole supports.
Terms associated with corona voltages are:
-corona inception voltage
-disruptive critical voltage
Showing posts with label Power Transmission. Show all posts
Showing posts with label Power Transmission. Show all posts
Friday, February 26, 2010
Tuesday, January 19, 2010
Power System Protection
Fault Vs Abnormal condition:
Abnormal condition in a power system, as the word itself states, is a non-desirable state in a power system.
Best example is 'Overload' in a power system, where the load current is above the nominal rating of the system it is designed for.
Whereas Fault is an abnormal condition which is detrimental to the power system , and will be dangerous if not eliminated.
Best example is a short-circuit, whereby two phase conductors touch each other between the load and the generator - so that a very high current flows in the circuit.
Transformer getting fire due to fault
Image from: www.seeclab.com
Sequence analysis:
Balanced system is where in a 3-phase power system - the current flowing in all three phases are equal.
Unbalanced system is where in a 3-phase power system - the current flowing in all three phases are unequal.
Any unbalanced system can be represented by balanced systems containing 3 elements. Those are,
1. +ve sequence
2. -ve sequence
3. zero sequence
Vector addition of the above 3 elements will result in the former unbalanced system.
Protection Equipments:
Fuse:
1. Semi-enclosed
-found in domestic installations, rewirable.
rewirable fuse holders
-found in electrical apparatus such as UPS etc, not re-wirable.
cartridge fuse
3. High Rupturing Capacity (HRC)
-normally found in the secondary (LV) side of transformers. This has high current capacity before breaking.
Knife-edged Low volatge HRC fuse links
Drop-down-lift-operate (DDLO) fuse:
These are sometimes called fusible cutouts.
This is a type of expulsion fuse, which is normally found in the HV side of transformers. When high current flows in the primary side (HV) of transformers - the fusing element (a special metal wire) will melt so that the connection will be cut-off due to gravity.
a DDLO in opened position
Miniature circuit breaker (MCB)
This is the protective device seen in modern homes, replacing older fuses. available in low current versions.
for e.g. 6A, 10 A, 16A etc. These MCBs normally have two tripping phenomena. one is magnetic coil used for instantaneous tripping ,and thermal bimetallic strips used for inverse-time overcurrent tripping.
These are available in different classes - considering the load current characteristics such as high inrush (starting) currents. High starting currents are caused when loads such as fluorescent lamps and motors are switched on.
single pole , three pole , four pole versions are available depending on the number of wires.
Moulded case circuit breaker (MCCB)
These are high current versions of domestic MCBs. Normally found in factories, utility bulk supply (3-phase) entry points.
3-phase currents in the orders of even 500A can be handled by these MCCBs.
Earth leakage circuit breaker (ELCB)
This is the older version of RCCBs. These devices detect the leakage current to earth and trip if that current exceeds a threshold, for e.g. 30mA, 100mA.
It should be noted that ELCB is a (residual)-voltage-operated device.
Residual current Devices (RCD)/ Residual current circuit breaker (RCCB)
This is a modern version of an ELCB, which works by comparing the residual current (resultant) produced - by means of checking the current difference between live and neutral wires .
It should be noted that RCCB is a (residual)-current-operated device.
Protective Relays:
1. Overcurrent (O/C) relays
2. Overload (O/L) relays
3. Earthfault (E/F) relays
4. Under frequency (U/F) relays
5. Overvoltage (O/V) relays
6. Distance relays
7. Differential relays
8. Reverse power (R/P) relays
9. Bucholz relays
10. Directional relays
11. Overspeed (O/S) relays
Arcing Horns/ gaps:
These are sometimes employed in transformer HV terminals or between the terminals of an HV insulator, to protect them from lightning surges. This works on the simple principle of HV rod-gap breakdown.
High Voltage Circuit breakers:
This was earlier explained in the post - "Grid sub stations"
***
Saturday, January 02, 2010
Grid Sub-Stations (GSS)
This is an arrangement of electrical conductors, towers, protective equipment, transformers etc for the operation and maintenance of a power transmission network.
Most common equipments found in a GSS with few abbreviations are:
1. Power transformers
2. Circuit Breakers (CB)
3. Isolators
4. Current transformers (CT)
5. Potential transformers (PT)/ Capacitive voltage transformers (CVT)
6. Bus bars (BB)
7. Surge arrestors (SA)
8. Line trap (for PLC communication)
9. Earthing transformers
10. Auto-Recloser
11. Overhead earth wire
12. Underground earthing system
Power transformers:
When classified depending on the voltage levels in both sides of a transformer:
1. step-up type (used in voltage increase from alternator-> transmission line)
2. step-down type (used in voltage decrease from transmission line->distribution)
Classified depending on the insulation medium:
1. mineral oil-filled type
2. dry-type
Circuit Breakers :
These are normally classified according to the arc-quenching medium around the contacts:
1. Air circuit breakers; Air Blast circuit breakers (ACB)
2. Vacuum circuit breakers (VCB)
3. Oil circuit breakers (OCB)
4. Gas circuit breakers (eg: SF6 breakers)
Isolators:
These are mechanical devices used to open an electrical path. This is particularly vital as a visual indication of isolating high voltage components, which is not provided by a CB.
These can be operated only in Off-Load condition.
Current transformers:
A type of transformer used to reduce the magnitude of the flowing current in a conductor, so that current can be handled safely for measurement & instrumentation.
Primary side is the current measured and secondary side will have the reduced current. Reduction in magnitude will be determined by the turns ratio (e.g.: 400/5, 1000/5, 2000/5 etc. ).
110 kV High-voltage Current tranformer in a grid substation
It should be noted that the secondary side of a CT is NEVER open-circuited. This is to avoid the dangerous high voltage present in the secondary side of the CT.
for e.g. :
If a 400A/5A CT is used in a transmission line rated at 132kV & 400A, secondary voltage will become 132*400/5 kV (= 10,560 kV) – if it is left open circuited.
Potential transformers:
A type of transformer used to reduce the magnitude of the voltage in a conductor, so that voltage can be handled safely for measurement & instrumentation.
Like CTs, reduction in voltage will be determined by the turns ratio of a PT.
Busbars:
These are normally made of hollow Copper/Aluminium rods. The reason is to account for the high current flow so that normal cables would be unable to withstand the electrical stress produced.
Busbars as seen in a GSS
Surge arrestors/ Lightning arrestor:
These are devices made for the protection of a power system arising from dangerous surges. These surges (high voltage impulses of shorter duration) are either from lightning or load switching.
These SA’s work allow the normal power frequency waves (50 / 60 Hz) but yield a grounding to surges (have very high frequency in the order of 10000 Hz).
Gas Insulated Sub-Stations (GIS):
A modern development is to make grid substation indoors. introduction of SF6 gas as insulation medium with very desirable charctersitics.
A gas-insulated indoor substation by ABB Inc
These type of substations occupy little space compared to the conventional outdoor stations, making them useful in densely populated cities or harshly-polluted areas.
For a deatiled study visit wikipedia.
Flow of equipments in a Typical Grid SubStation in USA
Image from: www.osha.gov
Image from: www.osha.gov
Most common equipments found in a GSS with few abbreviations are:
1. Power transformers
2. Circuit Breakers (CB)
3. Isolators
4. Current transformers (CT)
5. Potential transformers (PT)/ Capacitive voltage transformers (CVT)
6. Bus bars (BB)
7. Surge arrestors (SA)
8. Line trap (for PLC communication)
9. Earthing transformers
10. Auto-Recloser
11. Overhead earth wire
12. Underground earthing system
Power transformers:
When classified depending on the voltage levels in both sides of a transformer:
1. step-up type (used in voltage increase from alternator-> transmission line)
2. step-down type (used in voltage decrease from transmission line->distribution)
Hermetically sealed Mineral oil-filled transformer
Image from: www.toppowertransformer.com
Image from: www.toppowertransformer.com
Classified depending on the insulation medium:
1. mineral oil-filled type
2. dry-type
Circuit Breakers :
These are normally classified according to the arc-quenching medium around the contacts:
SF6-filled High Voltage Gas Circuit Breaker
Image from: www.made-in-china.com
Image from: www.made-in-china.com
1. Air circuit breakers; Air Blast circuit breakers (ACB)
2. Vacuum circuit breakers (VCB)
3. Oil circuit breakers (OCB)
4. Gas circuit breakers (eg: SF6 breakers)
Isolators:
These are mechanical devices used to open an electrical path. This is particularly vital as a visual indication of isolating high voltage components, which is not provided by a CB.
Air-break Isolator
Image from: www.yashexports.tradeindia.com
Image from: www.yashexports.tradeindia.com
These can be operated only in Off-Load condition.
Current transformers:
A type of transformer used to reduce the magnitude of the flowing current in a conductor, so that current can be handled safely for measurement & instrumentation.
Primary side is the current measured and secondary side will have the reduced current. Reduction in magnitude will be determined by the turns ratio (e.g.: 400/5, 1000/5, 2000/5 etc. ).
110 kV High-voltage Current tranformer in a grid substation
It should be noted that the secondary side of a CT is NEVER open-circuited. This is to avoid the dangerous high voltage present in the secondary side of the CT.
for e.g. :
If a 400A/5A CT is used in a transmission line rated at 132kV & 400A, secondary voltage will become 132*400/5 kV (= 10,560 kV) – if it is left open circuited.
Potential transformers:
A type of transformer used to reduce the magnitude of the voltage in a conductor, so that voltage can be handled safely for measurement & instrumentation.
Like CTs, reduction in voltage will be determined by the turns ratio of a PT.
Busbars:
These are normally made of hollow Copper/Aluminium rods. The reason is to account for the high current flow so that normal cables would be unable to withstand the electrical stress produced.
Busbars as seen in a GSS
Surge arrestors/ Lightning arrestor:
These are devices made for the protection of a power system arising from dangerous surges. These surges (high voltage impulses of shorter duration) are either from lightning or load switching.
Surge/lightning arrestor
Image from: www.electrical-res.comThese SA’s work allow the normal power frequency waves (50 / 60 Hz) but yield a grounding to surges (have very high frequency in the order of 10000 Hz).
Gas Insulated Sub-Stations (GIS):
A modern development is to make grid substation indoors. introduction of SF6 gas as insulation medium with very desirable charctersitics.
A gas-insulated indoor substation by ABB Inc
These type of substations occupy little space compared to the conventional outdoor stations, making them useful in densely populated cities or harshly-polluted areas.
For a deatiled study visit wikipedia.
Saturday, December 12, 2009
An introduction: Basic electricity and power engineering
Electricity is a form of energy, at the same time, a power source. It is an energy source when it is captured for a length of time and a power source when it is seen in a momentary time (instantaneous).
Normally electricity is produced where a form of energy is converted into another form of energy.
eg: 1. Chemical energy -> Electrical energy : Wet and dry cell batteries
2. Mechanical energy -> Electrical energy : Turbine generators
3. Light energy -> Electrical energy : Solar Photovoltaic panels
In another point of view, there are lot of methods electricity is produced in commercial scale. those are,
- Steam power plants
- Gas power plants
- Diesel power plants
- Bio mass power plants
- Geo thermal power plants
- Solar power plants
- Wind power plants
- Nuclear power plants
etc.
For a long time, electricty was seen as flow of positive energy particles through a completed circuit. But later it was found out that it is the 'free electrons' which contribute to the flow of 'charges'. That is why conventinally current is denoted in the direction of + to - in a 1.5 V dry cell battery - whereas the electrons flow in the opposite direction.
(note that flow of negatively charged particles ASSOCIATED with the imaginary flow of positive paticles called 'holes' is the true representation of the electricity)
Electrical parameters - Unit
1. Voltage difference - Volts
2. Current -Amperes
3. Power - Watts*
* there are other units as well, with different meanings- of electrical power.
A perfect analogy of current flowing in a conductor is, similar to water flowing in a sland (sloped) pipe,
Where the waterflow is like the electric current; voltage difference is like the slope of pipe and the electric power is the work done on flowing water per unit time.
When electric current is flowing in a conductor, it creates magnetic field lines along the conductor. When a voltage is present in a node (a point), it creates electric field lines around that node. Theoretically speaking, electromagnetic wave is made up of electric and magnetic fields which are perpedicular- propagating in a direction under certain circumstances. But a simple study of basic electricity is different from the complex study and mathematics of this field interaction in an electromagnetic wave. so this topic will not be discussed here now.
When considering the electrical conductivity of materials, common materials are devided into 3 categories.
1. conductors* (copper, aluminium, gold, carbon rod etc)
2. insulators (mica, poly-ethene, plastics, porcelain etc)
3. semi-conductors (doped silicon, doped germanium)
* At present, there is a further classification of 'super conductors'. Some special materials - in extreme circumstances like very low temperature have superconductivity; a state where electrical resistance is almost negligible. This has again wonderful applications (like semi conductors) in the field, for eg., superconductivity is used to obatain ultra high powerful electro-magnets used in electric trains.
Note that pure Silicon is an insulator. They become semi-conductors if they are doped with other elements like Boron or phosporous. These materials are of utmost importance in the electronics industry and revolutionised the world by the discovery of 'Semiconductor Transistors' in the Bell laboratories.
Current in a circuit can be direct or alternating. It is called direct (dc) when the polarity or direction of the current does not change with time.It is called alternating (ac) when the polarity does change with time in a definite manner such as changing 50 times per second. This leads to the 50 Hz electrical supply. In most parts of Americas, the supply will change it's polarity 60 times (60 Hz).
An ac supply can be of single phase (2/3 wires) or polyphased. The most popular polyphase power system is three-phase(3/4 wires) electricity supply. A three phase system is more economical than seperate single phase systems.
In a utility's (power company) point of view, a power system is seen in three major steps;
1. Power generation
2. Power transmission
3. Power distribution.
In a consumers point of view, a power system is seen as 'Power utilisation', which includes supply reliability, supply quality and energy efficiency.
The domestic power supply (in Sri Lanka) has the following characteristics.
1. Supply voltage is at 230 V ac
2. The incoming supply has two ac terminals: Live and Neutral wires
3. In addition, there is a ground/earth wire installed in the home premises.
4. Supply frequency is 50 Hz.
The major electrical machines involved in a power system are,
1. Generators
2. Motors
3. Transformers
Generators are further classified into ac, dc generators; while motors are classified into synchronous, asynchronous motors; And transformers can be classified into step up, step down transformers.
Normally electricity is produced where a form of energy is converted into another form of energy.
eg: 1. Chemical energy -> Electrical energy : Wet and dry cell batteries
2. Mechanical energy -> Electrical energy : Turbine generators
3. Light energy -> Electrical energy : Solar Photovoltaic panels
In another point of view, there are lot of methods electricity is produced in commercial scale. those are,
- Steam power plants
- Gas power plants
- Diesel power plants
- Bio mass power plants
- Geo thermal power plants
- Solar power plants
- Wind power plants
- Nuclear power plants
etc.
For a long time, electricty was seen as flow of positive energy particles through a completed circuit. But later it was found out that it is the 'free electrons' which contribute to the flow of 'charges'. That is why conventinally current is denoted in the direction of + to - in a 1.5 V dry cell battery - whereas the electrons flow in the opposite direction.
(note that flow of negatively charged particles ASSOCIATED with the imaginary flow of positive paticles called 'holes' is the true representation of the electricity)
Electrical parameters - Unit
1. Voltage difference - Volts
2. Current -Amperes
3. Power - Watts*
* there are other units as well, with different meanings- of electrical power.
A perfect analogy of current flowing in a conductor is, similar to water flowing in a sland (sloped) pipe,
Where the waterflow is like the electric current; voltage difference is like the slope of pipe and the electric power is the work done on flowing water per unit time.
When electric current is flowing in a conductor, it creates magnetic field lines along the conductor. When a voltage is present in a node (a point), it creates electric field lines around that node. Theoretically speaking, electromagnetic wave is made up of electric and magnetic fields which are perpedicular- propagating in a direction under certain circumstances. But a simple study of basic electricity is different from the complex study and mathematics of this field interaction in an electromagnetic wave. so this topic will not be discussed here now.
When considering the electrical conductivity of materials, common materials are devided into 3 categories.
1. conductors* (copper, aluminium, gold, carbon rod etc)
2. insulators (mica, poly-ethene, plastics, porcelain etc)
3. semi-conductors (doped silicon, doped germanium)
* At present, there is a further classification of 'super conductors'. Some special materials - in extreme circumstances like very low temperature have superconductivity; a state where electrical resistance is almost negligible. This has again wonderful applications (like semi conductors) in the field, for eg., superconductivity is used to obatain ultra high powerful electro-magnets used in electric trains.
Note that pure Silicon is an insulator. They become semi-conductors if they are doped with other elements like Boron or phosporous. These materials are of utmost importance in the electronics industry and revolutionised the world by the discovery of 'Semiconductor Transistors' in the Bell laboratories.
Current in a circuit can be direct or alternating. It is called direct (dc) when the polarity or direction of the current does not change with time.It is called alternating (ac) when the polarity does change with time in a definite manner such as changing 50 times per second. This leads to the 50 Hz electrical supply. In most parts of Americas, the supply will change it's polarity 60 times (60 Hz).
An ac supply can be of single phase (2/3 wires) or polyphased. The most popular polyphase power system is three-phase(3/4 wires) electricity supply. A three phase system is more economical than seperate single phase systems.
In a utility's (power company) point of view, a power system is seen in three major steps;
1. Power generation
2. Power transmission
3. Power distribution.
In a consumers point of view, a power system is seen as 'Power utilisation', which includes supply reliability, supply quality and energy efficiency.
The domestic power supply (in Sri Lanka) has the following characteristics.
1. Supply voltage is at 230 V ac
2. The incoming supply has two ac terminals: Live and Neutral wires
3. In addition, there is a ground/earth wire installed in the home premises.
4. Supply frequency is 50 Hz.
The major electrical machines involved in a power system are,
1. Generators
2. Motors
3. Transformers
Generators are further classified into ac, dc generators; while motors are classified into synchronous, asynchronous motors; And transformers can be classified into step up, step down transformers.
