News - Energy from Rubber Sheets

Engineers at Princeton University have developed rubber films fitted with piezo-electric elements which, they say, could generate power from ordinary activities like walking, breathing, etc.

Piezo- electricity refers to the phenomenon of some materials to develop an emf when they are subjected to a mechanical force.  The silicone rubber sheets have ceramic nanoribbons embedded onto them.  The ceramic material used is lead zirconate titanate, a piezoelectric material which can convert 80% of the mechanical energy into electricity, a higher efficiency than quartz.   The lead zirconate titanate, also known as PZT, is made into nanoribbons so small that 100 of these can be fitted into a millimeter

Possible applications include doormats which can generate power when someone walks over them.  Shoes with piezo-electric soles which can generate power with every step.  Applications in the field of medicine include power supplies for pacemakers in heart patients.  Pacemakers which are implanted in the body require power supplies which are surgically fitted inside the body.  This include regular surgical operations to renew the power supplies.  Piezo-electric sheets which can be implanted inside the body can be used to power the pacemakers from the force exerted during breathing.

(Image Courtesy: Frank Wojciechowski/Princeton University)

Earthing in Potential Transformers

The Secondary of the potential Transformers are usually earthed.  There are various reasons for this.  The first reason is concerned with safety.  Since, the primary winding of the potential transformer is connected to High voltage, there are chances for the high voltage, in rare conditions, to get transferred to the secondary.  This can occur due to capacitance, insulation failure or any other damage.  Such a scenario would be extremely dangerous.  For this reason, the secondary is usually earthed at the star point for star connected transformers.







Another reason for earthing the secondary would be functional.  For applications such as synchronization where two PT voltages need to be compared, the synchronizing equipment (dark lamp method and bright lamp methods) would function only if the star point of the PTs are grounded.

Neutral Inversion in Ungrounded Three phase systems

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.

Overvoltage - Causes and Protection

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 in Sub-stations

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

Calculating the percentage impedance of a Transformer

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.

Open Delta Connection of PTs

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.