PCC Transmission Poles

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 Poles

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 - Construction

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 Pit Construction

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.

Load Forecasting

Load Forecasting is an important function in power system operation.  Load forecasting is projecting the estimated load in advance.  Load forecasting can be done for a range of time periods from a few hours to many years. 

Load forecasting is done to decide which power generating units need to be taken in line in a certain period of time.  This is done to determine the best mix of generating units which will give the lowest cost of generation. 

The maintenance activities of the various power plants are also planned based on the load forecasting.  Investments for new power plants which may be required a few years down the line are also planned based on the load forecasting.

The load forecasting is done using historical data and factors, such as seasonal variations, projected economic activity, population growth, etc

Shunt Capacitors and Series Capacitors - A Comparison

Shunt Capacitors	Series Capacitors
Shunt Capacitors are used to improve the power factor of the system.	They are used to offset the effect of the series reactance in the line.
Shunt capacitors are not effective in situations where the line reactance of the line is high.	Series capacitors are useful in conditions of high series reactance.
Shunt Capacitors are connected as a single bank connected to the bus or individually near the loads.	Series Capacitors are connected in series with the load

Classification of Bus Bars in Load Flow Analysis

A bus is a set of conductors in which electrical power flows and is then distributed to different lines.
In Power Flow Analysis, buses are classified into three main types.

They are

• The Generator Bus
• The Load Bus and the 
• Slack, Swing or Reference bus

The Generator Bus
As the name suggests, the generator bus is the bus producing power.  For the generator bus, the values of P, the active power and the voltage, V are given.  The reactive power, Q and the load angle, delta should be found.

The Load Bus
The load bus consumes active and reactive power.  For this bus, the active power, P and the reactive power, Q are given.  The voltage and the phase angle should be calculated.

The Swing Bus
The swing bus is the bus which has been made to take the additional active and reactive power which are used to supply the transmission losses.  Therefore, the active and reactive powers are unknown for this bus.  Only the voltage,V and the load angle, delta are given.

Bus Type	Quantities Specified	Quantities to be found
Generator Bus	P,V	Q, load angle
Load Bus	P,Q	V, load angle
Swing Bus	V, load angle	P,Q

Static Compensators - An Overview

Static Compensators are electric circuits which are used to compensate the surge impedance in a circuit.  They are also used for load compensation.  Static compensators are used to maintain a constant voltage during varying loads.  They are also used during sudden reduction in the loads or during tripping of the source. 

Static Compensators improve the power factor and,consequently, the system stability

A simple static compensator consists of an variable inductor and a capacitor connected in parallel.  The reactive power in the system can thus be compensated using these two elements.

Thyristors are often used in series with the inductor and compensator to precisely control the reactor power flow to and from the static compensator.