ELECTRICAL INSULATION
Electrical Insulation is a type of resistive layer. It offers very high resistance, much more than a conductor and semiconductor. At rated conductor’s voltage, no current flows through it. Insulation’s are used as a layer around potential carrying conductors. They are also used as a medium, a separation, or as a base for conductive elements.
Ceramic, Glass, PVC, Bakelite are some of the materials used in insulation.
NEED OF INSULATION
Conductor is an element which offers very low resistance while Insulation provides very high resistance. Current easily passes through the conductor while through Insulation, no current flows under the rated voltage.
Conductor and insulation are opposite to each other. Conductors seem more useful than insulation. But the thing is; without insulation, Conductor is nothing.
It is the insulation which arrange the provision that current only flows in defined path through the conductor. It is beneficial only when current passes through a defined path; Other-wise chances of danger & damage will be there.
That’s why insulation is equally important !
Image attached above shows the configuration of HT Cable
High voltage needs high safety. The Insulation around the conductor in HT cable is made up from thermosetting material which is crosslinked polyethylene(XLPE). The layer of XLPE insulation around the conductor is very thick But insulation thickness doesn’t guarantee the safety. A potential always creates an electric field stress around the conductor. High potential leads to high electrical field stress around the conductor.
High electrical stress around the conductor can damage the cable insulation itself. An additional internal layer of earthing is provided around the XLPE insulation. This is done for the safety of the cable, humans, and equipment. This layer neutralizes the electrical field stress around the conductor.
Each core has XLPE insulation coated with semi-conductive paint. A copper sheet is wrapped around that paint in a helix shape. In the cable termination process, the copper sheet from all the cores are earthed. The metallic Armour is also grounded from both the open ends of the cable. Earthing around each core diminish the Electric field stress & interaction in between the cores.
In 3 core HT cable, All the three cores are round that’s why Void get created in between the cores. To fill that void In between the cable cores; round PVC filler is provided to fill the void. This filler make the cable equally round from all the sides. After that, Inner sheath is wrapped around upon which metallic Armour is provided which provides mechanical strength to the cable. At last, outer sheath is provided.
APPLICATIONS OF INSULATION
The insulation’s property of offering the high resistance; is used in many different ways. Insulation is used as a layer, as a separation or as a medium or as a base for conductive elements. All these applications are going to be discussed one by one, next !
Insulation As a layer around the conductors
As discussed above without insulation, Conductor is dangerous. It is the Insulation which confines the current in defined direction & assures proper safety. Also to arrange the multiple conductors in a compact size such that multi-core cables, Insulation is the key !
An image of 2.5mm² 4Core is shown. Each core contains insulation around it. All the 4 cores are centered in a PVC sheath.
Insulation As a separation
To separate multiple conductors, insulation papers are placed between them. This ensures effective separation. This method is particularly useful in motor winding.
An attached image shows how winding coils are been sandwiched in between the F class insulation paper.
Insulation As an insulating Medium
Insulation is used as an insulating medium around the conductive elements. Mineral oil in Transformers acts as an insulating medium around the winding. It also functions as a cooling medium.
Insulation as an Insulating base for PCB
Electronics is the heart of technology. In Electronics, Micro components such as IC’s, resistances, capacitors & inductors are arranged on PCB(Printed Circuit Board).
PCB is a type of insulated plate. A conductive circuit is printed on it. Multiple micro components are embedded on it. Again, Behind the arrangement of multiple micro components on compact space with most possible way, Insulation is the key!
IMPACT OF HIGH VOLTAGE ON INSULATION
Under the voltage rating, insulation completely restricts the flow of current. However, higher voltage beyond a certain limit can damage the insulation. Current can then pass easily through the insulation. That voltage is known as breakdown voltage for that insulation.
A high voltage much more than the rated capacity of the system excites the insulation at its molecular level. This results in the breakdown of insulation resistance, and current starts flowing through the insulation.
A TEST report of Transformer Oil is attached below Which is been performed on a Oil of 500KVA_11KV/415V Transformer. That report shows the breakdown voltage or B.D.V voltage of 40KV at 2.5mm gap. Which means That transformer’s oil provides proper insulation to 11KV Line voltage While at 40KV at potential Gap of 2.5mm gap, Oil’s insulation get breaks & Starts conducting.
INSULATION MEASUREMENT
Voltage & resistance, Both are the numerical values which can go beyond infinity. High voltage requires high insulation. Insulation of particular system depend upon its voltage rating. Insulation value is been defined with its resistance value which is having the units ‘ohm‘.
Special devices such that analog or digital insulation testers are used to measure the Insulation resistance. These insulation measuring devices produces the high voltage DC which is been generated around the test specimen. A fixed DC voltage produces an resistance dependent current when applied on a test specimen. Through a fixed voltage & measured current, resistance value been calculated.
A Gif below shows the connections & working of Insulation Tester In which Insulation of motor’s winding is checked. One terminal of the Insulation tester connects to the earth terminal of the Motor. Another terminal of the device connects to one terminal of the motor. After pressing the test button, Meter shows the reading.
Insulation resistance is a numerical value which has no end. To measure a particular resistance range at higher accuracy, a preferred voltage range is used. An insulation resistance measurement range by fluke is discussed below. This explains the best test voltage range for insulation resistance range.
An attached image shows :
At a test voltage of 250VDC, the measurable range for ±5% accuracy is 200KΩ to 5GΩ. For ±20% accuracy, the measurable range is 5GΩ to 50GΩ.
AT a test voltage of 500VDC, the measurable range for ±5% accuracy is 500KΩ to 10GΩ. For ±20% accuracy, the measurable range is 10GΩ to 100GΩ.
SUM UP
- Electrical Insulation is a type of resistive layer Which offers very high resistance more than conductors & semiconductors.
- Insulation’s are used as a layer around the potential carrying conductors. They serve as a medium or a separation. They also act as a base for conductive elements.
- Ceramic, Glass, PVC, Bakelite are some of the materials used in insulation.
- Insulation & Conductors, Both works Hand in Hand; Both are equally important.
- Under the voltage rating, insulation completely restricts the flow of current. A higher voltage beyond a certain limit can damage the insulation. The current can then pass easily through it. That voltage is known as the breakdown voltage for that insulation.
- Special devices such as meggar & digital insulation testers are used to measure insulation resistances.
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