Neutral Inversion is a phenomenon in which the neutral of an ungrounded three phase system falls out of the voltage triangle

This usually occurs in a three phase system which is not earthed and where a single transformer is used in a voltage-based earth-fault sensing system.  The transformer may be connected to one of the phases and the ground.  This transformer needs to be provided without sufficient resistance loading.  The no-load or exciting impedance of the transformer is in parallel with the line capacitance of the phase to which the transformer is connected. 

Hence, if the exciting impedance is more it the ratio of the capacitive impedance to the inductive impedance decreases causing a higher voltage in the neutral.  This high neutral can sometimes shift outside the triangle.

This  situation can be avoided by providing sufficient resistance loading to the secondary of the voltage transformer.  This reduces the inductive impedance of the transformer and limits the shifting neutral voltage.




Overvoltages occur in a system when the system voltage rises over 110% of the nominal rated voltage.  Overvoltage can be caused by a number of reasons, sudden reduction in loads, switching of transient loads, lightning strikes, failure of control equipment such as voltage regulators, neutral displacement,.  Overvoltage can cause damage to components connected to the power supply and lead to insulation failure, damage to electronic components, heating, flashovers, etc.

Overvoltage relays can be used to identify overvoltages and isolate equipment.  These relays operate when the measured voltage exceeds a predetermined set-point.  The voltage is usually measured using a Potential Transformers.  The details of the ratio of the potential transformer are also entered into the relay.  These relays are usually provided with a time delay.  The time delay can be either instantaneous, fixed time or for IDMT (inverse definite minimum time) curves.

Generally, overvoltage relays are provided with sufficient time delay in order to avoid unwanted trippings due to transients (See article on Transients).

These relays can be used to isolate feeders and other equipment connected to the network.  In the case of generators, these relay also switch off the excitation system to the generators thereby preventing voltage build-up. 




Thermography is an extremely useful tool in the maintenance of substations. Thermography helps monitor the components in the substation. Using temperature-based imaging, it identifies hot spots which can indicate potential problems. Thermography is an integral part of predictive maintenance schedule. Thermography involves monitoring devices with a thermal imaging camera. Thermal imaging cameras operate on the infrared spectrum which is invisible to the human eye. The images formed by these devices show variations in temperature in the object. These variations in temperature can be interpreted as abnormalities and analyzed.

Thermal cameras work by measuring the wavelength of the radiation emitted by hot bodies. The wavelength of the radiation depends on the temperature of the equipment.

Most electrical problems such as loose connections, overloading, etc are accompanied by a rise in temperature. This rise in temperature can give timely warnings, which, if heeded can avoid major failures and breakdowns.

Thermography is usually carried out in the early mornings to enable clearer differentiations of the hot spots from the surrounding temperature. The components which are being inspected should be in the normally loaded condition.

Inspection should start from the top of the equipment and proceed downwards. This avoids omissions of any part of the equipment. It is ideal to establish inspection routes in the substation, through which the engineer moves across with the imaging camera. This ensures that all areas of the substation are covered.

Thermal images of all components should be recorded and temperatures noted. The temperatures of two similar components carrying similar loads should not differ by more than 17 degrees and the difference between any component and the surrounding air(ambient temperature) should not exceed 40 degrees.

If any anomaly is detected, the thermal image should be recorded along with a ordinary photograph. It is advisable to take the thermal images from different angles to obtain a good perspective.

images courtesy: www.imaging1.com




The impedance of a transformer is defined as the percentage of the drop in voltage to the at full load to the rated voltage of the transformer.  This drop in voltage is due to the winding resistance and leakage reactance.


Alternatively, the percentage of a transformer can be described as the percentage of the nominal voltage in the primary that is required to circulate the rated current in the secondary.

The impedance of a transformer can be measured by means of a short-circuit test.

The secondary of the transformer whose percentage impedance is to be measured is shorted.  The voltage on the primary is gradually increased from zero till the secondary current reaches the transformer's rated value. 

The percentage impedance of the transformer is calculated as

Z%= (Impedance Voltage/Rated Voltage)*100

Thus a transformer with a primary rating of 110V which requires a voltage of 10V to circulate the rated current in the short-circuited secondary would have an impedance of 9%.

The percentage impedance of a transformer a crucial parameter when operating transformers in parallel. It also determines the fault level of a system during faults.




The Open Delta Connection is used to connect PTs. The connection uses two transformers one on each phase. The output current, though, is only 57.5 percent of a normal three phase connection. The capacity is consequently reduced to 86.6% of a normal three phase configuration.

The Open Delta connection can be used where one transformer of a three phase PT assembly has failed.

The open delta connection is cheaper that a conventional delta connected PT. Another advantage of the open delta connection is that it can be used service one transformer in a connection while the system runs on the other two.




Float charging is used where the battery rarely gets discharged.  A typical application where float charging can be used would consist of the float charger, battery and the load in parallel.  During normal operation, the load draws the power from the charger.  When the supply to the charger is interrupted, thebattery steps in.

Float charging of a battery involves charging the battery at a reduced voltage.  This reduced voltage reduces the possibility of overcharging.The Float charger ensures that the battery is always in the charged condition and is therefore considered "floating".  The Float charger starts by applying a charging volltage to the battery.  As the battery gets charged, its charging current reduces gradually.  The float charger senses the reduction in charging current and reduces the charging voltage.

If the battery gets drained, the float charger will again increase the charging voltage and process continues.  Float chargers can be connected indefinitely to the batteries.

Boost charging involves a high current for short period of time to charge the battery.  It is generally if the battery has been discharged heavily.  Boost charge enables the quick charging of depleted batteries.

For instance, a two volt lead acid battery which has been discharged will initially be boost charged with a charging voltage of around 2.35-2.4 volts.  However, as the battery voltage rises, the charger will switch over to the float charge mode with a float voltage of 2.25 volts.

Most battery chargers come equipped with provisions for both boost and float charging.




Single phasing is a condition in three phase motors and transformers wherein the supply to one of the phases is cut off. Single phasing causes negative phase sequence components in the voltage. Since, motors generally have low impedances for negative phase sequence voltage. The distortion in terms of negative phase sequence current will be substantial.

Negative phase sequence currents cause heating of the motor and consequently motor failure.

Single phasing is caused by the use of single-phase protection devices such as fuses and circuit breakers. Three phase loads should be protected by devices which cause the interruption of power to all three phases simultaneously when a fault occurs.

Defective contacts in three phase breakers can also cause single phasing.

Single phasing can be identified by special protective relays which can identify and isolate the connected loads. Smaller motors rely on overcurrent and negative phase sequence relays. Motor protection relays for larger motors come readily fitted with protection against single phasing.

Single phasing can sometimes cause excessive noise and vibration in motors.




In 1897, the German Physicist Peukert proposed that as the rate of a lead-acid battery's discharge increases its available capacity.This is known as Peukert's law.  It can be Mathematically represented as

Cp=Ikt

where Cp is the capacity of the battery in ampere hours
k is the Peukert's constant
I is the current and
t is the time

The Peukert's Constant k is specified by the manufacturer of the battery and is usually in the range of 1.2 to 2.

Thus the time a battery can provide sustain a certain current without any appreciable drop in voltage would be given by 

t=Cp/Ik

Thus a 100Ah capacity with a Peukert's constant of 1.2 will be able to supply a current of 5A for 14.5 hours.




The electrolyte in the battery is a mixture of sulphuric acid and water.  The amount of water in a battery can fall due to electrolysis or evaporation.  This may cause in a drop in the level of the electrolyte and consequently a drop in the battery output. 

Hence, it is necessary to periodically inspect the level of electrolyte in the battery. If the level of the electrolyte falls below the minimum level, it can be topped up by adding water.  Only distilled water should be added as ordinary water may contain a lot of impurities and ions which may contaminate the electrolyte.


The level of electrolyte in the battery tends to fall as the battery gets discharged and tends to rise as the battery gets charged.  Hence, water should be added to the electrolyte only when the battery is fully charged.  If the water is added to the battery when it is in the discharged condition, the level can increase beyond the limit when the battery is fully charged and may overflow

The acid used as the electrolyte is extremely corrosive and should be handled with extreme care.  Proper protective outfits should be worn while handling them.  Water can be added to a container of acid.  However,  acid can never be added to container of water as the heat generated can cause splashing. 





Specific Gravity of electrolyte refers to the its relative density.  Specific gravity is the ratio of the density of a liquid to the density of water.  The specific gravity is measured by means of a hydrometer.  The specific gravity gives an indication of the amount of charge in a battery.

When a lead acid battery is charged, the sulphuric acid which is the electrolyte is transformed into water.  The specific gravity of the electrolyte varies between 1.1 and 1.3.

The specific gravity should be periodically checked.  If the specific gravity becomes more than 1.3, the electrolyte may be overly acidic and can damage the plates.  If the specific gravity is less than 1.1, the plates can become hydrated.

The specific gravity is directly linked with the open circuit voltage (OCV) of the battery.  The open circuit voltage rises and falls with the specific gravity of the electrolyte.

Individual manufacturers give a graph or a table describe the exact relationship between the open circuit voltage and specific gravity.

Specific gravity of the electrolyte also varies in accordance with temperature, it decreases with increase in temperature and increases in colder conditions.




Back Flashovers generally occur in transmission lines during lightning strikes when the potential of the tower rises vis-a-vis the conductor.  This causes the voltage across the insulators to increase beyond the limits resulting in a flashover.

Lightning strokes have the ability to discharge thousands of amperes of current in very short time. This high current needs to be discharged quickly into the earth to prevent the potential of the tower from rising.

Back flashover occurs when the lightning which has struck the tower is unable to get discharged to the earth.  This can occur due to high impulse resistance of the ground.

When a tower is struck by lightning, a travelling voltage is induced which moves many times between the top and the bottom of the tower , the potential of the tower is thus raised.  The elevated voltage also appears on the cross arms of the towers.  This can cause back flashovers if the insulators are unable to withstand the voltage surge.

Back flashovers can be avoided by improving the impulse resistance of the earth point of the transmission towers and improving critical flashover limits of the insulators.

Back flashovers are identified as line to earth faults.






For the 20 percent wind scenario to work, billions must be spent on installing wind towers on land and sea and about 22,000 miles of new high-tech power lines to carry the electricity to cities, according to the study from the Energy Department's National Renewable Energy Laboratory.

"Twenty percent wind is an ambitious goal," said David Corbus, the project manager for the study. "We can bring more wind power online, but if we don't have the proper infrastructure to move that power around, it's like buying a hybrid car and leaving it in the garage,"

The private sector cannot fund all the needed spending, so a big chunk would have to come from the federal government through programs such as loan guarantees, Corbus said.

The Obama administration is already dedicating billions of dollars to double the amount of electricity produced by wind and other renewables energy sources by January 2012.

The Interior Department will decide this spring whether to approve the Cape Wind project off Cape Cod, Massachusetts. That project, long delayed because of local opposition, would provide electricity to about 400,000 homes.

The amount of U.S. electricity generated by wind was up 29 percent during January-October of last year compared to the same period is 2008, according to the Energy Department.

Reaching the 20 percent threshold for wind by 2024 in the eastern electric grid would require 225,000 megawatts of wind generation capacity in the region, about a 10-fold increase from current levels, the study said.

One megawatt of electricity can provide power to about 1,000 homes.

Wind turbines would be scattered throughout the eastern grid, which extends from the Plains states to the Atlantic Coast and south to the Gulf of Mexico.

Most of the big wind farms would be concentrated off the Atlantic Coast in federal waters from Massachusetts to North Carolina and on land in Midwest states from North Dakota to Nebraska and into Kansas.

Many states already require utilities to produce a portion of their electricity from renewable energy sources, but a federal mandate covering all utilities nationwide would help create the 20 percent wind scenario, Corbus said.

Sen. Byron Dorgan said on Tuesday he thought the Senate would forgo dealing with climate change legislation this year after going through the contentious health care debate and instead focus on passing an energy bill that, in part, requires U.S. utilities to generate 15 percent of their electricity from renewables by 2021.

source: reuters.com





Extech Instruments have launched a Multimeter, the CAT IV, with a Wireless PC interface.  The device also comes with in-built datalogging facility.  The devices transmits measured data using Radio waves.  A special receiving device is provided which needs to be connected to the USB port of your laptop. 

Wirelss contact enables handsfree communication.  The device provides True RMS function for all current and voltage measurements.  It comes with a double-molded body confirming to IP 67.  The backlit display
provides for simultaneous display of voltage and frequency.  All functions are provided with 1000V input protection. 

The device has an accuracy of 0.06%.  Dual Frequency sensitivity frequency function is available with an electrical range of 40 to 400Hz and an electronic range of .001 to 100Mhz.  The device weighs around 350 grams.