Deep Cycle Batteries

Deep Cycle Batteries are batteries which can be discharged to very low levels.  These batteries are generally used where continuous use of the battery is required such as  in battery operated vehicles such as golf carts, floor sweepers and forklifts.  

The deep cycle battery differs from other batteries in its construction. The plates of the battery  are thicker as compared to the sponge type plates of other batteries.  

The use of solid plates with lower surface area  in the deep cycle battery means that the battery cannot give  sudden pulses of high current as may be required for batteries used in applications vehicle starting.  The deep cycle battery is designed to provide a high current over a long period. 

Deep Cycle batteries are widely used in the field of renewable energy.  They can be used to store energy from solar panels during the day and can be discharged during the night.  Deep Cycle batteries usually have a higher C rating (refer article on C rating).  This enables them to be discharged over many hours.

Peak Load Shaving

Peak Load Shaving refers to operating certain generators specifically to address peak demands in the load cycle of a utility.   The load cycle tends to peak in the mornings and falls during the noon.  It again rises in the evening and the night.

Managing these peaks in the load cycle is an important function of distribution systems. Peak Load Shaving, as this process is known, is done by starting certain power sources specifically to offset this peak demand.  These are usually sources with higher generating costs like diesel generator sets.  These power sources are run only during periods of peak loads or in times of emergency.

Another method of peak load shaving is by providing incentives to customers to reduce consumption during times of peak demands.  This levels the load cycle curve and enables optimum loading of power sources. 

Synchronous Compensators

Synchronous Compensators are synchronous motors which are run without any load.  These motors are used to generate or absorb reactive power from the system.  Generally located near large loads, these motors, running in the overexcited condition provide reactive power according the the load demand. 

In large transmission lines, the line capacitances generate excessive capacitive line charging current which can lead to a rise of voltage in the system. In such a scenario, the synchronous compensator can run underexcited and absorb the excess reactive power.  This is particularly true in EHV (Extra high voltage systems) where the capacitive line charging is higher than the magnetizing power of the load even during loaded conditions. 

While static reactors and capacitors can be used in reactive power regulation in lines, sychronous compensators have the advantage of quick responses and fine control depending on the excitation. 

In newer installations, static compensators with thyristor controls have replaced synchronous compensators owing to their low maintenance and running costs.

Thyrite Arrestors

Thyrite is a material obtained by a special type of clay mixed with carborundum (Silicon Carbide).  Thyrite is used widely in lightning arresters.  Thyrite is a non-linear resistor. i.e. it has high resistance at low voltages and low resistance at high voltages.  A two times increase in voltage causes the current to increase by nearly 12 times.  Hence, heavy currents can be discharged during voltage strikes and other surges.  This heavy discharge of current enables quick reduction of the surge voltage preventing flashovers. 

The Thyrite arresters are usually arranged in parallel with the primary winding of the Transformer.  The Thyrite inside the arrester is arranged in the form of discs.  The sides of the discs are metal plated to decrease resistance between the discs.  

Once the surge has been discharged, the Thyrite quickly returns to its high resistance state.

Slow Blow Fuses

Slow Blow fuses are fuses which have an inherent time delay.  These fuses are widely used in motor protection circuits.  These fuses help discriminate between the high inrush current of motors and a high current caused by a fault. 

The time delay in the fuse is achieved by designing the fuse element with a higher mass for the same current rating.  This higher mass causes the element to heat gradually in the event of an overcurrent.  Thus, the fuse element takes time to heat and melt to isolate the current.

Kraft Paper Insulation

Kraft Paper is a special paper used in electrical insulation.  Kraft paper is just plain paper from which the alkaline salts have been removed.  It is usually impregnated with mineral oil, jelly or wax to improve its dielectric properties.  

Kraft paper is made thermally resistant by applying a diamond patterned epoxy resin to both sides.  This is known as thermally upgraded Kraft paper which finds wide application in oil filled transformers for coil layer insulation. 

Paper capacitors are also made using Kraft paper as the dielectric. 

How a Fluorescent Light Works - Schematic Animation

Water cooled busbars

Water cooled bus bars are used in induction heating applications where the bus bar carrying currents is subjected to extreme temperature.  The currents used in induction heating is usually very high of the order of kilo amperes.  

Water cooled bus bars are also used to build power electromagnets which require very high currents.  Such electromagnets are generally used for research purposes. 

In such hot environments, the bus bar which is carrying heavy current should be continually cooled to prevent it from melting due to the external heat.  Demineralized water is circulated through holes drilled through the bus bars.  

The temperature of the water should be maintained below the boiling point of water.  This is because the water can flash to steam above the boiling point and the high pressure created can damage the bus bar. 

Corrosion is a issue in water cooled bus bars.  The water quality should be constantly monitored to prevent corrosion. 

Turbo Alternators

Turbo Alternators are alternators coupled to turbines.  Turbo alternators run at high speeds and develop large quantities of power usually of the order of hundreds of Megawatts.  The rotor of the Turbo alternator is usually made of many pieces connected together.  Turbo generators usually have cylindrical rotors.  The speed of the rotor is around 3000 rpm (50 Hz) or 3600 (60 Hz). 

Turbo alternators are extremely sensitive machines as they operate at very high speed.  The slots for the rotor windings are milled on the rotor shaft.  The rotor of the turbo alternator does not have damper windings as the prime mover used is usually a turbine which provides a consistent and steady torque unlike the diesel engine which provides pulsating torque.   The wedges used to secure the rotor winding to the slots are made of steel and function as a damper winding as well. 

Cooling is a major issue in Turbo generator, given the enormous amount of heat generated.  Air, Hydrogen and water have been employed to transfer the heat away from the generator. 

What is Pull-in torque and Pull-out Torque in synchronous machines

A synchronous motor is not self starting.  Thus, its speed has to be brought near the rated rpm before it can run on its own.  

There are many methods of starting the synchronous motors such as the use of an external pony motor, amortisseur windings which start the motor as an induction motor and the use of VFDs. 

When the speed of the synchronous motor reaches 97% of the nominal speed, the dc supply to rotor of the synchronous motor is switched on.  This produces a "pull-in" torque which helps the rotor poles to catch the imaginary stator poles and run in synchronism. 

The "pull out" torque of the synchronous motor is the torque which can cause the motor to slip and step out of synchronism.  In some cases, the rotor misses a pole and catches the next stator pole.  In many other cases, the rotor runs like the rotor of an induction motor and never gains synchronism.  This can result in serious damage to the rotor.  Hence, synchronous motors should be provided with a reliable "pull-out" protection