Negative resistance is a phenomenon seen in many non-linear electronic devices.  In this phenomenon, the current increases as the voltage decreases and vice versa.

This is in opposition to Ohm's law, which states that the current should vary linearly with the voltage.  This phenomenon is seen in electronic components, such as the Gunn diode, the tunnel diode and thyristors.

The fluorescent tube light is an example for negative resistance, when conduction starts, the current rises to a high level.  This is accompanied by a drop in voltage, causing the negative resistance.  To prevent the high current from damaging the device, an inductance (ballast) is connected in series.

There are no negative resistance components, such as resistors.  Certain devices exhibit negative resistance in a particular region of the VI curve.

Power system stability is the stability of a system to respond to disturbances in the system.  Power system stability is also known as synchronous stability.  Power system stability is classified into three types.

They are

  • Steady state stability
  • Transient stability
  • Dynamic stability

Steady state stability is the stability of the system to respond during minor disturbances during normal operations.  The variation in the bus voltages and the phase angles are relatively small.  Steady state stability is caused by switching small loads.

Transient stability, on the other hand, is the analysis of the response of the system to sudden and large variations in the system voltage and phase angles.  This kind of variation is caused by a sudden fault or overload, caused by a tripping of a power source.

Power flow studies need to be conducted to study the response of a power system to steady state and transient faults.

Dynamic stability is the stability which is maintained against small variations.  These variations can coalesce into large variations which can cause large fluctuations and tripping.  Dynamic stability is ensured by the use of sophisticated control equipments.

Bentonite is a naturally occurring clay.  It is used to reduce the earth resistance in earth pits.  Bentonite has the property of absorbing water. 

Bentonite is mixed with ordinary clay and placed between the earthing conductor and the earth.  Water is poured into the mixture.  Bentonite expands to many times it size as it absorbs water.

The earth resistance is reduced due to the water absorbed by Bentonite.  Bentonite will also absorb rain water and retain it for longer than the surrounding soil.  Bentonite earth pits may require water to be poured into the pit during dry seasons.  Special funnels are usually provided in these earth pits for this purpose.

The downsides are that Bentonite has many corrosive impurities, which may corrode the earthing conductors.  This may require periodic maintenance of the earth pits.

PCC (Prestressed Concrete Poles) are poles used in the tranmission and distribution of power.

PCC poles are made by placing high tensile wires in moulds.  The wires are stressed by stretching mechanisms which pull the wires apart.  When the wires are in this stressed condition, concrete is poured into the mould.  After the concrete sets, the wires are released.

A galvanized wire is run through the metal frame to provide for earthing.  This wire is connected to the earthing network.

The concrete is thus prestressed.  This kind of concrete can withstand impact and has high tensile strength.

PCC poles are more expensive than RCC poles.  They are aesthetically pleasing.  They can be manufactured at the site itself.  This will save transportation costs.

RCC (Reinforced Concrete Poles) are widely used in distribution system.  They are made of concrete with a metal reinforcement.

RCC poles are used as their are aesthetically pleasing and blend in well in urban areas with streets with buildings.

RCC poles are cheaper.  They can also be constructed at the site itself.  This reduces the cost of transportation.

RCC poles also have a long life and require less maintenance.  Sometimes, the concrete may chip and become porous.  In such conditions, water may percolate through the concreate and reach the metal reinforcement.  This can cause corrosion.

A downside is that RCC poles tend to shatter when a vehicle collides with them.  This can be prevented with the use of PCC (Prestressed Cement Concrete) poles.

Earth mat is a method of earthing,  It is used in areas with rocky soil, where it is difficult to excavate earth pits.  The earth resistance obtained with pits will also be above the required value.

Earth mats obtain the earth resistance with a large contact area.  The earth mats consists of a number of flat strips laid in the form of a grid.  The grid is placed in a trench which is about 75 cms deep.  The flats are made of mild steel and are of 75 x 8 mm size. 

The flats are welded to one another.   The low resistance is achieved by the increased contact area this method provides.

The earth resistance should ideally be less than 1 ohm.  The resistance value should be periodically checked and recorded.

Earth pits are a very widely used method of earthing electric systems.  Earth pits provide a very low earth resistance and are very reliable.

The earth pit is constructed as follows:

Excavate a pit of approximately the following dimensions 70cms x 70 cms x 250 cms. 

Place a cast iron pipe electrode that is 2.5 metres long and of size 75 -125 mm diameter.  The pipe should be about 10 mms thick.

The pipe is placed vertically. 

A mixture of Bentonite and black cotton soil is mixed in the ratio of 1:6.  The pipe is filled with this mixture.  The space surrounding the pipe is also filled with this mixture.

The earth pit is an important part of any earthing scheme.  It should be maintained properly at regular intervals.  Earth resistance measurements should be taken periodically.