electrical engineering

Ionization of Gases

<div dir="ltr" style="text-align: left;" trbidi="on"><div style="color: red;"><span style="font-size: large;">Ionization increases rapidly with voltage once these secondary processes take place, until ultimately breakdown occurs</span></div><br />
<br />
<div style="color: blue;"><span style="font-size: large;">1 / Ionization processes in gas discharges</span></div><br />
<div style="color: red;">The electrical breakdown of a gas is brought about by various processes of ionization. <br />
These are gas processes involving the collision of electrons, ions and photons with</div><div style="color: red;"> gas molecules, and electrode processes which take place at or near the electrode surface. </div><br />
<span style="font-size: large;"><span style="color: blue;">Relevant gas ionization processes</span></span><br />
<div style="color: red;"><span style="font-size: small;"><br />
</span></div><span style="color: red; font-size: small;">In the case of simple collision, the neutral gas molecule does not always get ionized on electron impact.<br />
In such cases, the molecule will be left in an excited state M*, with energy Ee</span><br />
<br />
<span style="font-size: large;"><b><span style="color: blue;">Ionization by Double electron impact</span></b></span><br />
<div style="color: red;"><br />
</div><span style="color: red;">If a gas molecule is already raised to an excited state (with energy Ee) by a previous collision, then ionization of this excited molecule can occur by a collision with a relatively slow electron</span></div>

Ionization of Gases

Ionization increases rapidly with voltage once these secondary processes take place, until ultimately breakdown occurs 1 / Ionization processes in gas discharges The electrical breakdown of a gas is b…

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07 Dec 2010

Base load and peak load

<div dir="ltr" style="text-align: left;" trbidi="on"><span style="color: red; font-size: large;">• In the case of using only one power plant, the<br />
capacity of the power generation will be defined<br />
according to the peak value<br />
• The plant will operate at a part of load for prolonged<br />
time, which decreases the efficiency<br />
• Several smaller units are installed with some of them<br />
covering base load and others covering peak load<br />
• The units that cover the base load operate<br />
continuously, while those taking the peak load<br />
operate when required<br />
• All units operate near their rated power with high<br />
efficiency</span><br />
<br />
<span style="font-size: large;"><span style="color: red;">The generating units that operate as base power</span><br style="color: red;" /><span style="color: red;">plants should have the following characteristics:</span><br style="color: red;" /><span style="color: red;">• Low operating cost since they operate</span><br style="color: red;" /><span style="color: red;">continuously</span><br style="color: red;" /><span style="color: red;">• High capability of operating continuously for long</span><br style="color: red;" /><span style="color: red;">time</span><br style="color: red;" /><span style="color: red;">• Low and fast maintenance requirements</span><br style="color: red;" /><span style="color: red;">• The load factor is very high and reaches unity for</span><br style="color: red;" /><span style="color: red;">many units</span><br style="color: red;" /><span style="color: red;">Thermal, nuclear and hydraulic power plants are</span><br style="color: red;" /><span style="color: red;">conventionally considered as base load plants</span></span><br />
<div style="color: red;"><span style="font-size: large;"><br />
</span></div><span style="color: red; font-size: large;">The units that operate as peak power plants should<br />
have the following characteristics:<br />
• Very fast response to load variation (high ramp rate)<br />
• Low startup time<br />
• Low start up cost<br />
• Low capital cost for economic operation since they<br />
operate for relatively short time during the year<br />
• Constant voltage and frequency against the load<br />
fluctuations<br />
• The load factor is very low in the range of 0.1-0.6<br />
Steam, gas turbine and diesel power plants are<br />
used as peak power plants</span><br />
<br />
</div>

Base load and peak load

• In the case of using only one power plant, the capacity of the power generation will be defined according to the peak value • The plant will operate at a part of load for prolonged time, which decre…

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07 Dec 2010

Reserve power part 2

<div dir="ltr" style="text-align: left;" trbidi="on"><div style="color: red;"><a href="http://hayatei.blogspot.com/2010/12/reserve-power.html"><span style="font-size: large;"> Reserve power part 1</span></a><br />
<br />
<br />
<span style="font-size: large;">The choice of the operating and standby unit as well</span></div><div style="color: red;"><span style="font-size: large;">as the distribution of reserve power on different power</span></div><div style="color: red;"><span style="font-size: large;">plants depends on the following factors:</span></div><div style="color: red;"><span style="font-size: large;">The start up cost: depends on the start up time “the</span></div><div style="color: red;"><span style="font-size: large;">time interval between the order of starting to the</span></div><div style="color: red;"><span style="font-size: large;">moment where the unit delivers power to the network”</span></div><div style="color: red;"><span style="font-size: large;">Units with high start up cost have not to be frequently</span></div><div style="color: red;"><span style="font-size: large;">switched on and off. Such units can provide hot</span></div><div style="color: red;"><span style="font-size: large;">reserve power but not cold reserve power.</span></div><div style="color: red;"><br />
</div><span style="color: red; font-size: large;">The shut down cost: the cost due to the wasted<br />
energy during the shut down of the unit. It depends<br />
also on the type of the unit<br />
Ramp rate of the unit: the increase of power that<br />
can be obtained from a generating unit in a specific<br />
time. The power can not be increased or decreased<br />
suddenly but it needs a certain time per kW. This<br />
prevents the effective use of slow units (low ramp<br />
rate) for spinning reserve.<br />
The total reserve power is not obtained from a single<br />
unit because this would take a long time. Rather, the<br />
spinning reserve is distributed over the available<br />
units to get the required power in a short time.</span> </div>

Reserve power part 2

Reserve power part 1 The choice of the operating and standby unit as wellas the distribution of reserve power on different powerplants depends on the following factors:The start up cost: depends on t…

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07 Dec 2010

Reserve power

<div dir="ltr" style="text-align: left;" trbidi="on"><div style="color: red;"><span style="color: red; font-size: large;">The power system is continually subjected to<br />
unexpected load changes<br />
The generating units supply about 85-90% of<br />
their capacity maintaining the remainder for<br />
emergencies<br />
Some units are kept in the hot status to be<br />
connected to the network as fast as possible</span></div><div style="color: red;"><span style="font-size: large;"><br />
</span></div><span style="font-size: large;"><span style="color: red;">The need for reserve power arises due to some</span><br style="color: red;" /><span style="color: red;">abnormal conditions in power system</span><br style="color: red;" /><span style="color: red;">Sudden unexpected increase in the load demand</span><br style="color: red;" /><span style="color: red;">Forced outage of generators or other equipments</span><br style="color: red;" /><span style="color: red;">due to stability problems</span><br style="color: red;" /><span style="color: red;">Underestimating the load demand due to some</span><br style="color: red;" /><span style="color: red;">errors in the load forecasting</span><br style="color: red;" /><span style="color: red;">Local shortage in the generated power (e.g. due</span><br style="color: red;" /><span style="color: red;">to the outage of a transmission line)</span></span><br />
<div style="color: red;"><span style="font-size: large;"><br />
</span></div><span style="color: red; font-size: large;">The choice of the operating and standby unit as well<br />
as the distribution of reserve power on different power<br />
plants depends on the following factors:<br />
The start up cost: depends on the start up time “the<br />
time interval between the order of starting to the<br />
moment where the unit delivers power to the network”<br />
Units with high start up cost have not to be frequently<br />
switched on and off. Such units can provide hot<br />
reserve power but not cold reserve power.</span><br />
<br />
</div>

Reserve power

The power system is continually subjected to unexpected load changes The generating units supply about 85-90% of their capacity maintaining the remainder for emergencies Some units are kept in the hot…

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07 Dec 2010

What is Buoyancy

<div dir="ltr" style="text-align: left;" trbidi="on"><span lang="EN-US" style="font-family: "Californian FB";"><span style="font-size: x-small;">In simple terms, buoyancy is the ability of an object to float. </span></span><div><span style="font-family: "Californian FB";"><span style="left: -3.56%; position: absolute; top: 0.74em;"><span style="font-size: x-small;">•</span></span></span><span lang="EN-US" style="font-family: "Californian FB";"><span style="font-size: x-small;">Whether or not a given object will float is determined by the buoyant force on the object. </span></span></div><div><span style="font-family: "Californian FB";"><span style="left: -3.28%; position: absolute; top: 0.74em;"><span style="font-size: x-small;">•</span></span></span><span lang="EN-US" style="font-family: "Californian FB";"><span style="font-size: x-small;">This force is caused by the difference between the pressure at the top of the object, which pushes </span></span><span lang="EN-US" style="font-family: "Californian FB";"><span style="font-size: x-small;">it downward and the pressure at the bottom of the object which pushes it upward.  Because the </span></span><span lang="EN-US" style="font-family: "Californian FB";"><span style="font-size: x-small;">pressure at the bottom of the object is always greater than the pressure at the top, every </span></span><span lang="EN-US" style="font-family: "Californian FB";"><span style="font-size: x-small;">submerged object feels an upward buoyant force. </span></span></div><div><span style="font-family: "Californian FB";"><span style="left: -3.39%; position: absolute; top: 0.74em;"><span style="font-size: x-small;">•</span></span></span><span lang="EN-US" style="font-family: "Californian FB";"><span style="font-size: x-small;">The magnitude of this buoyant force is equal to the weight of the fluid that is displaced by the </span></span><span lang="EN-US" style="font-family: "Californian FB";"><span style="font-size: x-small;">submerged object. </span></span></div><div><style>
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</style><o:shapelayout v:ext="edit"></o:shapelayout><o:idmap data="1" v:ext="edit"></o:idmap><p:colorscheme colors="#0099cc,#eaeaea,#003366,#66ffff,#33ccff,#9999ff,#cc99ff,#008080"></p:colorscheme><div class="O" v:shape="_x0000_s1026"><div><span style="color: white; font-family: "Californian FB";"><span style="color: #66ffff; font-size: 55%; left: -3.46%; position: absolute; top: 0.74em;"><span style="font-size: x-small;">•</span></span></span><span lang="EN-US" style="color: white; font-family: "Californian FB";"><span style="font-size: x-small;"><b>The difference between the buoyant force acting up on the object and the force due to </b></span></span><span lang="EN-US" style="color: white; font-family: "Californian FB";"><span style="font-size: x-small;"><b>gravity acting downward on the object will determine whether or not the object will </b></span></span><span lang="EN-US" style="color: white; font-family: "Californian FB";"><span style="font-size: x-small;"><b>float or sink. </b></span></span></div><div><span style="color: white; font-family: "Californian FB";"><span style="color: #66ffff; font-size: 55%; left: -3.48%; position: absolute; top: 0.74em;"><span style="font-size: x-small;">•</span></span></span><span lang="EN-US" style="color: white; font-family: "Californian FB";"><span style="font-size: x-small;"><b>If the buoyant force is greater than the gravitational force, the object will float, but if </b></span></span><span lang="EN-US" style="color: white; font-family: "Californian FB";"><span style="font-size: x-small;"><b>the opposite is true, the object will sink.</b></span></span></div></div></div></div>

What is Buoyancy

In simple terms, buoyancy is the ability of an object to float. •Whether or not a given object will float is determined by the buoyant force on the object. •This force is caused by the difference betw…

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07 Dec 2010

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