Line transients and spikes are the main factors causing the wrong operation, erratic performance and inaccurate recording of the sensitive and sophisticated systems. Some of the effects of harmonics are:
- Voltage harmonics cause an increase in iron losses, current harmonics cause an increase in copper loss ans stray flux losses and increased heating of the transformers.
- Unacceptable system voltages & over-currents cause premature insulation failure.
- Create mechanical stress by creating mechanical oscillations in the rotating machinery.
- Power capacitor failure.
- Over loading on the cables.
- Malfunction of ripple control system.
- Nuisance tripping of protection relays.
- Malfunction and failure of computer and loads.
- Heavy interference in the Digital logic system.
As day by day devices are becoming more sophisticated, the clean power requirement has become even more stringent. To avoid these and ensure clean power supply to the sensitive equipment isolation transformers can be used. In addition to this they isolate the load from the mains.
PRINCIPLE OF OPERATION: The schematic diagram of isolation transformer is shown in figure below:
This transformer is capable of attenuating the passage of line disturbance/noise from its primary winding to the secondary winding. Here faraday electrostatic shield are used between the primary and secondary to reduce the coupling capacitance between primary and secondary. These attenuates noise in both directions IN as well as OUT. It attenuates common mode noise (between ground wire and current carrying conductors) as well as transverse mode noise (between current carrying conductors).
The basic principle of operation is that any variation in the voltage causes an induced voltage on the shielding (copper shields, which are effectively earthed) and these voltages are grounded to the earth. The good earth is of vital importance for effective protection by Isolation transformer. The isolation transformers can protect the equipment against voltages as high as 2.5 kV for about 2 minutes. To further enhance the protection, over-voltage arrestors can be introduced before the isolation transformer.
CONSTRUCTION DETAILS: The good isolation transformers (called ultra or super) employs unique multiple shielding technique to reduce the inter-winding capacitance to below 0.005 pF, whereas an ordinary isolation transformer may have inter-winding capacitance to the order of 50 pF. As a result these Isolation transformers have only about ten thousandth of the leakage current compared to the ordinary Isolation transformers. The leakage current is limited to about 20 μA. The split windings and bi-filliar construction of windings are used to reduce the coupling capacitance.
The insulating material used shall be of high grade. Superior quality core and proper size of copper and good shielding technique shall be used for long life, less losses and trouble free service.
For three phase systems, three single phase units are connected in Star configuration.
- To protect computers, CNC machines and Telecom equipment from noise, spike and minor surges.
- To protect against lighting, impulse noise, short circuit, accidental discharge of capacitors.
- To protect medical equipment where leakage current is critical.
- To shield a large number of electronic equipment which individually produce different type of noises on common bus, and also to reduce damage due to the circulating noise interference.
- To protect equipment where ground cannot be earthed.
SPECIFICATIONS: The typical specification for an Isolation transformer are given below:
|Capacity:||30 kVA, 3 phase.|
|Input Voltage:||415/440 V 3 phase.|
|Output Voltage:||415/440 V 3 phase (Taps at ±5 %)|
|Ratio:||1 : 1.|
|Load regulation:||Better than ±3.5 %.|
|PF:||0.75 lag to 0.75 lead.|
|Dielectric Strength||2500 V for 120 s.|
|Insulation Resistance:||Greater than 1000 MΩ.|
|Coupling Capacitance:||Less than 0.05 pF.|
|Leakage Current:||Less than 20 μA.|
|Common mode attenuation:||More than 100 dB.|
|Conformity to standards:||As per IS 2026 Pt I & II.|
|Ambient temperature:||Up to 50 °C.|
|Enclosure:||Made out of minimum 16 SWG MS sheet.|
|Indications:||LED type indicating lamps in respective colours.|