SuperCapacitors - an Overview

Super capacitors are capacitors with very high capacitance values sometimes reaching up to 500 Farads.  Supercapacitors are used to store energy just like batteries.  The principle of a supercapacitor is similar to that of a capacitor.  However, the supercapacitor is built using nano-technology.  This enables the dielectric to have a very large surface area and thus store greater quantity of charge. The electrodes are usually made of activated charcoal while the dielectric is an Electrolyte soaked separator. 

Supercapacitors are advantageous over batteries in that they are lighter, more environmentally friendly and can be recycled.  Besides, supercapacitors can be  charged and discharged repeatedly unlike batteries.

Supercapacitors are also known as Ultracapacitors or Electolytic Double Layer Capacitors.   The lifetime of a supercapacitor is can be upto 100 times the lifetime of a battery. 

Supercapacitors find uses in cameras, electric automobiles, power conditioners, welders.

Supercapacitors are used as power sources in conjunction with batteries.  Supercapacitors can supply short burst of power and are useful when heavy loads are applied suddenly.  They also charge faster and absorb voltage transients better while the battery supplies the regular power requirement. 

The are also used in automobiles to where they can be charged easily and are particularly effective in recovering energy from the transmission systems through regenerative braking.

The downside of supercapacitors is the low energy to weight ratio as compared to batteries.  Further advances in technology may narrow out this difference. 

Discharging Capacitors

Capacitors are widely used in Electrical and Electronic applications.  They are used in filter circuits, for power factor correction, for starting motors and for a host of other applications.  Capacitors store electric charge and can retain it for long periods, even days and weeks.  Hence, they can cause a severe electric shock or burns when someone accidentally makes contact with them even when the equipment has been switched off.

Thus, when an equipment is being repaired, it is essential to ensure that the capacitors inside are safely discharged. 

There are three common ways of discharging capacitors.  The first is by shorting the leads with a metallic object such as a screw driver or a wire.  This method causes a rapid discharge of the capacitor as the leads are shorted.  This results in excessive current which can melt the leads of the capacitor.  The molten metal can be thrown around the equipment damaging components and even causing damage to people nearby (in the eyes and skin).  Hence, this method should not be used.

The second method is to discharge the capacitors with filament lamps.  By connecting a filament lamp of the appropriate voltage lamps, the capacitor can be discharged.  The lamps glow initially due to the presence of the charge.  The light, then slowly diminishes indicating the discharge of the potential.  If the voltage is higher than the voltage rating of the lamp, two or more lamps can be connected in series. 

The third method of discharging capacitors is by the use of resistors.  A resistor with a suitable wattage rating can gradually discharge the capacitor.  This is the most ideal method of discharging capacitors.   (See also article on Bleeder Resistors)

Tropicalization of Electric Equipment

Protecting Electric Equipment from the surrounding atmospheric conditions such as humidity, heat, cold, fungal attack, salt spray.   etc is known as the tropicalization of Electric Equipment.  Electric Equipment such as circuit breakers, contactors, PCBs are all tropicalized.  The level of Tropicalization is determined by the environment where the equipment is going to be fitted.   

One of the methods of tropicalization is known as conformal coating.  Conformal coating is done by spraying a dielectric material on the surface of the equipment to provide protection against the ingress of moisture, fungi and other elements.  Substances such as acrylic, silicone, urethane, epoxy can be used in the coating. 

Circuit Breaker Operating Time Measurement

The operating time of a circuit breaker is crucial in any protection scheme.  A circuit breaker that takes too long to open will seriously compromise protection causing damage to equipment and people.

Hence, circuit breakers should be periodically tested to see whether they operate at the correct operating time specified by the manufacturer.

Being mechanical devices, circuit breaker are made of numerous springs, washers, and linkages.  These components can get jammed, the lubricating oil in the linkages can evaporate or lose its properties.  The lubricating oil can mix with dust and form a viscous deposit.  This can cause partial seizure and  is particularly true for circuit breakers which are kept in the closed or open position for very long periods of time.   

The Circuit Breaker operating time can be measured by special testing equipment known as the Time interval meters.  These instruments measure the time between the signal to open  and the actual interruption of the current.    The opening time of the contacts belonging to each individual phase is measured.

Arc Flash Protection

An Arc Flash is a serious accident in which a short circuit or an earth fault creates an arc.  The tremendous heat generated can cause severe burns and even death to people who are working nearby. (See Article on Arc Flash).  Arc Flash Relays seek to minimize the damage caused by an arc.  The damage due to an arc can be isolated if the isolating circuit breakers feeding the fault are made to trip at as soon as possible.

An arc which lasts for about 500ms can cause severe damage to people and equipment.  An arc lasting 100ms will cause lesser damage.  An arc lasting just 35 ms will cause far lesser damage. 

Normal protection schemes involving over current and earth fault relays will not be fast enough to isolate these faults.  Besides, making these relays more sensitive can cause undesirable false trippings. 

Arc Flash Relays work by sensing the light intensity caused during the flash over.  An arc causes a tremendous increase in the light intensity, often of the order of several thousand lux.  The light intensity in a normal office lighting will be around 200 or 300 lux.  The light sensed by the sensors is compared with a preset reference level.  The relay activates the trip signal if this reference is exceeded. 

Modern Arc Flash Relays can operate within 2.5 ms of a fault + the operating time of the circuit breaker.  The operating time of the breaker needs to be checked periodically.  Circuit breakers should open within 50ms of the command being given.  Delayed opening will seriously compromise protection. 

Air Break Switches

Air Break Switches are special switches designed isolate a circuit.  The are usually employed in out door installations.  Special Arcing Horns are provided to quench the arc which occurs when the current is interrupted.  These switches are usually operated by a handle which is located at the ground level.  Their operation can also be mechanised. 

Air Break Switches should not be used to interrupt load currents. They are isolating devices.   They can, however, be used to interrupt small currents such as the exciting current of a transformer or the capacitive charging current of a long transmission line.  A variation of the Air Break Switches is the Load Switch which can interrupt current on load as it has special arc quenching device. 

Air Break Switches can be mounted both horizontally and vertically.  These switches usually have a rugged construction as they are exposed to the weather.  Air Break Switches are usually found in groups of three, one for each phase.  These groups are opened together.  Hence, they are also known as gang-operated switches. 

Air Break Switches have been designed upto 135 kV

Jogging and Inching in Motors

Jogging refers to repeated starting and stopping of a motor in short bursts to perform a particular movement such as moving a crane to a particular location. The motor is operated on full power.  Special Jogging circuits are available. 

Inching is similar to Jogging and the two are often confused with each other.  Inching involves short thrusts of motion at reduced power.  The motor is made to run at a reduced voltage. 

Jogging should not be done more than 5 times in a minute.  This is because jogging can cause the motor to overheat.  If Jogging is to be done frequently, the starter of the motor should be derated. 
 
Special Jogging Blocks are also available which prevent the motor from being jogged more than 5 times an hour by logging the number of starts per hour. 

A simple jogging Circuit

Line Chokes

Line chokes are inductances which are connected in series to devices such as inverters and rectifiers.  Lines chokes are used widely in Drive systems, battery chargers and UPS.  They are available in both single phase and three phase designs.

Line Chokes reduce harmonics and limit surge currents.  They also protect rectifiers from voltage fluctuations in the mains supply (dv/dt phenomena).  Line chokes also reduce the peak currents drawn by inverters and reduce the starting current drawn by motors.