Current Limiting Fuses

Current limiting fuses are used in systems where high fault levels can result in excessive fault currents. The fuses function as normal fuses; however, they are designed to limit the fault current to low levels when they operate.

During normal operation, the fuse has a low resistance. However, when the fault occurs and the fuse ruptures, the heat created by the arcing inside the fuse causes the compacted quartz sand to create a high resistance environment. This quenches the arc and ensures that a very rapid fall in current.

The fault is thus cleared within the first half-cycle of the fault within 10 ms. Thus current-limiting fuses also protect systems from voltage sag in the event of a fault in one part of the system.

The current limiting fuses contains elements made of copper or silver. The elements are designed to have constrictions at a number of places which will heat up in the event of a fault. This enables quick operation. The arcing is also made to occur in a pre-determined number. The arcing which occurs in many streams enables easy quenching instead of one single arc. The quenching medium is usually compacted quartz sand.
Current limiting fuses also reduce hazards of arc-flashing, since they are extremely fast acting and also able to restrict the currents.

Arc flashing occurs when different conductors accidentally come into contact. The resulting arc can cause flashovers which generate tremendous amounts of heat causing danger to personnel nearby.

image courtesy :www.chfuses.com

Interharmonics

Interharmonics are distortions in the current or voltage waveforms, like ordinary harmonics. However, they differ from normal harmonics in that the frequency of these waveforms are not integral to the fundamental. The waveforms are found between the normal harmonics of voltage and current.

For instance, while normal harmonics have frequencies such as 150Hz (3rd Harmononic) and 250Hz(5th Harmonic), interharmonics appearing in frequencies between these harmonics such as 160 Hz, 198 Hz, etc. However, they can appear as discrete frequencies in a spectrum. Interharmonics are believed to be caused due to transient changes in the value of current and voltage.

They often accompany normal harmonics. Fast Changes in the phase angles of currents and voltages can also cause interharmonics. Another cause of interharmonics can be asynchronous switching of semiconductor switches such as in systems which use pulse width modulation. Arcing loads, such as welding machines and arc furnaces, are also believed to cause interharmonics.

The Effects of interharmonics are saturation of current transformers, disturbance of telecommunication signals, etc. Interharmonics are known to cause low frequency mechanical oscillations.

Filters can be used to mitigate the effects of interharmonics. However, factors such as resonance, power loss, etc need to be kept in mind while making the choice of filters. Series filters are generally used against interharmonics.

Subharmonics is a term used to refer to Harmonics which have a frequency less than the fundamental frequency i.e. 50 Hz.

Why is Gravel used to pave substation surfaces?

Gravel has many qualities which make it a preferred material for layering surfaces inside substations. Its high resistivity helps ensure that the step and touch potentials remain within limits.

It also prevents growth of weeds and small plants. It mitigates the chances of a fire in the event of oil spillage. It can be easily excavated. Besides, it also prevents the entry and movement of small animals and reptiles inside the substation.

Gravel also prevents the accumulation of water and the formation of puddles inside the substation.

All these features ensure that gravel is the material of choice for use in substations.

Water Trees and Electrical Trees and their role in cable insulation failure

Water trees are tree-like defects, filled with water, which develop in the insulation of cables. The defects usually originate from defects, voids or contaminants. The trees can cause premature failure of the insulation. Water trees usually propagate in the direction of the electric field. They occur only in the presence of water in the insulation. They are usually invisible to the naked eye in the dry condition. Special dying techniques are available which can make them noticeable.

Water trees are found more in sections of cables which are in a state of tension such as in bends. While it is possible to identify the conditions which may cause the formation of water trees, the exact mechanism and the chemical processes involved in their development is not yet fully understood. Water trees reduce the breakdown strength of the cable.

Electrical Trees are formed in the absence of water in dry conditions. They are caused by voids, impurities and defects in the insulation. High electrostatic stress which reverses direction as in AC cables can also accelerate the phenomenon. Occasionally, water trees may evolve into electrical trees. These trees are accompanied with partial discharge which may accelerate insulation failure. Electrical trees are readily visible to the naked eye.

Trees can be classified broadly into vented and bow-tie trees. Vented trees are those which originate from an electrode and reach out to another electrode. These trees grow faster as they have access to air which aids partial discharges.

Bow-tie trees are those that originate inside the insulation. Since they originate inside the insulation they do not have access to air and hence limited partial discharge occurs. They progress slower than vented trees.


image courtesy: http://www.lordconsulting.com

Earth Fault Monitoring in Ungrounded DC systems

Monitoring Earth Faults in Ungrounded dc systems is vital to prevent any sudden tripping in the system. In ungrounded DC systems, an earth fault in one terminal will not cause any disturbance and the system will continue to run normally. However, should an earth fault occur at the other terminal also, then there will be a virtual short-circuit between the two terminals through the earth.

Such faults occur without any indication and are difficult to identify. Hence a system to monitor the earth faults in a DC system is vital.

This is a schematic of a simple design of an earth fault monitoring system for ungrounded DC systems. The system consists of two bulbs. Each bulb is connected to one terminal of the power source and the ground. Thus, one bulb is connected between the anode and the ground while another is connected across the cathode and the ground as shown in the image.

Under normal terminals, the voltage across the bulbs will be zero and they will be off. However, in the event of a ground fault between the positive terminal and the earth, the voltage across the bulb connected to the cathode will be equal to the system voltage. This will cause the lamp to glow, indicating an earth fault.

Hipot Testing

Hipot Test is a high voltage test that is used to check the integrity of insulation for high voltage equipments such as busbars, cables, motors etc. The term 'Hipot' is the shortened form of High Potential. The Hipot test is used to ensure that an insulation can withstand a high potential without risk of failure.

However, the hipot test carries with it the potential failure of the insulation during the testing process itself. Weak insulation can fail during the test. Hence, many equipment owners avoid conducting this test. The hipot test certifies that the insulation is sufficient to withstand excess voltage during operation. This is significant in situations where the failure of a machine in service can cause serious damage or downtime as compared to a failure during the testing procedure.

The Hipot test is alternatively known as Dielectric Withstand test. The test involves the application of a high voltage usually about two times the
operating voltage. Thus a 6.6kV equipment will be tested at a voltage of 13kV.

The test is conducted for 1 minute or five minute. If the hipot test is conducted on a transformer winding or an alternator winding, the test is conducted on individual phases. The phases are separated and those phases which are not subjected to the hipot voltage are grounded.

Test Procedure
Prior to commencing the hipot test, it is necessary to get the Insulation Resistance and the Polarization Index values for the insulation. This ensures that the windings are free of any moisture or contamination. A wet or contaminated winding is more vulnerable to fail during the test.

The hipot test voltage is applied to the winding terminals to be tested. The voltage is sustained for one minute or five minute and then reduced. The current during the test period is also studied. Should there be a failure during the testing. There wil be a surge in the current which will cause the MCBs in the hipot test kit to trip.

There are two methods of raising the voltage to the value of the test voltage. They are

Step Test
In this method, the test voltage is raised gradually in small incremental steps. This enables the tester to abandon the test if he suspects that any current increase which may indicate a weak winding.

Ramp test
In this method, the test voltage is raised gradually or ramped up at a specific rate. The voltage can be increased to the rated voltage along with constant monitoring of the current. The ramp method is the most effective test as it can avoid any insulation failure during the test by identifying potential weaknesses in the winding early on.

The Hipot test does not offer scope for analysis such as the Insulation Resistance or the Polarization Test. It is simple a pass-fail kind of test. It is significant in that it gives operators the confidence that the equipment is strong enough to withstand the operating voltage and transient overvoltages in the system.

The high voltage used during the test calls for high standards of safety. The area around the test location should be cleared of all items not related to the test(clutter). The area needs to be cordoned off to prevent the entry of unauthorized persons. Personnel should be stationed at the main power switch so that the switch can be turned at the first sign of any abnormality. The personnel conducting the test should be properly trained with awareness of emergency first-aid procedures in the event of an electric shock. The device which is being tested should be grounded after the test to discharge the capacitance.

image source: http://www.testequipmentconnection.com

Electricity from Onions

An American company has come up with an ingenious way to generate electricity from the humble onion. According to Reuters, the new system is being installed at Gills Onions, the largest fresh onion processor in the U.S.
The system works by collecting the peels of the onions which are usually wasted. This waste is acted upon by special bacteria which generate methane. The methane is diverted to fuel cells which generate electricity. The electricity generated is estimated to be sufficient to power 360 homes.

See: http://in.reuters.com/article/entertainmentNews/idINIndia-41124320090717

What is the function of fuses in Instrument Transformers?

Instrument Transformers are sometimes provided with fuses. These fuses are intended to isolate the transformers in the event of internal fault.

Opinion is divided over the use of these fuses. Maloperation of these fuses can cause large scale disruption of power systems by way of unintended operation of the relays that are connected to the instrument transformers. Hence, some manufacturers provide fuses in the instrument transformers only when specifically requested.

While, the fuses cannot protect the instrument transformers from burnout during normal operation as the burnout current may be extremely low, they can ensure that a fault inside the transformers does not impact the busbar or generator in which it is mounted.