Sympathetic Tripping

Sympathetic Tripping refers to the phenomenon in Electrical Systems when a protective device in a healthy section of the system operates for a fault in another section of the system.  Sympathetic tripping results in unnecessary loss of power for healthy equipment. 

There are many causes for sympathetic tripping.  The most common reason is undervoltage which occurs across the system when there is a heavy current due to a short-circuit or an earth fault. 

Another reason for sympathetic tripping can be the flow of capacitive currents in the healthy feeders when one of the feeders gets grounded. 

In Transformers and Generators the Differential relay sometimes operates for an overcurrent which is outside its zone.  This is due to the dc component of the earth fault current.

Preventing Sympathetic Tripping   

Sympathetic Tripping can be prevented by designing smaller feeders with the total loads equally balanced across the different feeders

Reducing the fault level can result in lesser currents in the event of faults.  The fault level can be reduced by the use of current limiting reactors which increase the impedance.

Extreme Inverse settings in IDMT relays can also help the relays discriminate between sympathetic overcurrents and genuine faults. 

Increasing fault clearing times in the faulty feeders reduces the duration of the undervoltage across the system. 

Modern Differential relays have an inbuilt dc filter which prevent sympathetic tripping due to dc components during earth faults.

Videos on Power Transformer Testing

Useful Videos on Power Transformer Testing by OMICRON

Your Guide to Energy Saving Lighting

Lighting within the home currently accounts for about 8% of energy bills in the UK. Incandescent and tungsten bulbs have been a standard feature of electrical use since the 19th century, but are gradually being phased out in favour of more energy saving options that save consumers money and benefit the environment. A number of different options are available from online electrical wholesalers or high street retailers, from energy saving bulbs to compact fluorescents, LEDs and dimmers that can help to cut costs and generate more efficient energy. Moreover, these bulbs can be combined with a number of simple energy saving practices that can be followed within the home.

Types of Bulbs and their Benefits

Early incandescent and halogen bulbs relied on a tungsten filament, and remain the norm for most homes. Halogen bulbs are more efficient than incandescent forms, but still lag behind energy saving bulbs in terms of efficiency. The UK Government have promoted schemes to gradually phase out the use of older bulbs, while encouraging a switchover to energy saving lighting options. Energy saving bulbs and lights remains fairly expensive compared to older bulbs, but have the benefit of lasting longer, and reduce electricity bills.

A basic energy saving bulb is 5 times brighter than a standard bulb, and uses 80% less power. If used responsibly, some energy saving bulbs can have a 10 year life span. This length is based on using certain bulbs for three hours a day in parts of the home, and can be an ideal solution for rooms that are not used very often. Energy saving bulbs can result in 75-80% energy savings, and use 4 times less of the wattage of standard bulbs.

Other energy saving bulb options include compact fluorescents. These bulbs use an alternative gas charging method to standard bulbs, and use 20 to 25% less electricity. Again, higher costs for initial purchases can be offset by their long lasting potential and greater energy efficiency.

Another option is to invest in LEDs, or light emitting diodes. These represent strips of about 36 to 48 lights, which when installed can generate 50,000 hours of capacity. Representing 50 times as much capacity as a standard incandescent light, small LED arrays also act as a stylish alternative to hanging fittings, bulbs and lamps within rooms, and are particularly recommended for kitchens.

These lights emit less carbon dioxide than standard incandescent lights, and can consequently help conserve energy and the environment. When looking for energy saving bulbs always check for an Energy Saving Trust Recommended label, or an Energy Related A tag. Energy saving bulbs can also be recycled, and form part of the EU’s Waste Electrical and Electronic Equipment Initiative.

Other Tips

As well as investing in energy saving bulbs, you can also follow some simple steps when using lights in the home. The most basic solution remains turning lights off when they are not being used. Moreover, try to use lights for particular roles, with bulbs being turned off when watching a brightly lit television or computer. If reading, a single lamp is more efficient than keeping a whole room’s lights on. Dimmer switches are also useful in this regard for regulating the amount of light in a room, and can result in 4-9% of electricity savings.

About the author
Serena is a copywriter for a leading supplier of energy saving discount electrical supplies at Discount Electrical. In her spare time she writes various other blogs online on numerous other subjects such as automotive, health and the theatre.

Capacitor Trip Modules.

Capacitor Trip modules are used in breaker circuits to provide a source of back up power for trip operations in the event of the failure of the breaker control supply.  The capacitor Trip device is usually used in switchgear systems which use an AC control supply. 

If the control supply of a breaker panel fails during operation, the operator will not be able to operate the breakers from a remote location.  This would also compromise the protection scheme as the breaker will not trip even if a command is sent by a protection relay. 

The capacitor trip module consists of a capacitor which stores charge.  This charge is enough to trip the breaker for a certain number of times, say 5 times.  This ensures that the breaker can trip even if the control supply fails during operation.  Capacitor Trip modules are available for both AC and DC control systems. 

Grounding Transformers

Grounding Transformers are used in Ungrounded systems to provide a earth point.  Grounding Transformers are classified into two types

1) Zig Zag Transformers and

2) The Star-Delta Grounding Transformer with secondary unloaded.

We had looked at the Zig-zag Transformer in an earlier Post (Click here). 

Let us now look at the Star-Delta Grounding Transformer

The Star-delta grounding Transformer has a primary which is star connected and a delta secondary.  The phases of the star primary are connected to the busbar while the neutral is grounded.  The secondary of the transformer which is delta connected is usually left unloaded, though it can also be used to supply power.  The delta serves to provide a return flux path for unbalanced loads.  

During an earth fault, the zero sequence currents can flow through the grounded neutral of the transformer.  If the current is to be limited, a resistor can be added in series to the neutral of the transformer primary. 

Braking Resistors in Variable Frequency Drives

Braking Resistors are used in Variable Frequency Drives to dissipate the energy released by the motor into the power system.  The Braking Resistors perform the duty of absorbing the power from the rotor when the VFD reduces the speed to zero and preventing rotor heating.  They also prevent the rotor from exceeding the synchronous speed set by the Variable frequency drive.

A variable Frequency drive consists of three main components - the rectfier which converts the AC supply into DC, the DC busbars and the inverter which converts the DC supply into a variable AC supply.  The VFD varies the speed of the motor by varying the frequency of the AC supply applied at the motor terminals. 

When the motor is required to be stopped suddenly, the Variable frequency drive reduces the supply frequency to 0 HZ.  In this condition, the rotor is rotating at speed higher than the synchronous speed.  This causes the motor to behave like a generator and send power in the reverse direction, into the DC bus bars.  During this time, the voltage across the DC busbars can rise to very highlevels.  The braking resistors absorb power in this situation and prevent the voltage from rising beyond limit and damaging the Drive.     

The value of the resistances determines the rate of fall of the motor speed (braking). 

Special provision is made for cooling the resistors which can generate a huge amount of heat when in operation. 

Resistance Temperature Detectors

Resistance Temperature Detectors or RTDs are sensors which measure the temperature by altering their resistance. >The Resistance temperature detector consists of an element made of a metal such as platinum  located in a metallic casing.

When the temperature increases, the resistance of the sensor increases (positive temperature coefficient of resistance) This increase in the resistance is measured through a wheatstone bridge. The relationship between temperature and the resistance is linear.  Thus, the temperature can be deduced from the measured resistance.

Platinum and Nickel are two metals used to construct the sensing elements.  

Some common types of RTDs are the Pt-100 and Pt-1000. 

The Pt stands for Platinum while the number 100 stands for the ohmic value at 0 degrees Celsius.

The resistance increases linearly with temperature.

For example, the Pt100 has an ohmic value of 100 ohms at 0 °C and a value of 161 ohms at 160°C  

Advantages of RTDs

Accuracy

Long Term Stability

Ability to withstand shock and vibration

Disadvantages of RTDs 

Errors due to lead resistance,

Slow response

Internal Self heating

2 Wire, 3 wire and 4 wire RTDs

One of the disadvantages of the RTD is the error caused by the lead resistance.  That is, the indicating device which measures the sensor resistance to calculate resistance also measures the resistance of the leads connecting the sensor to the device.  This is unavoidable, though the error can be minimized by running a wire in parallel to one or both the leads. (Refer diagram)