Measurement of insulation resistance on a power transformer

Measurement of insulation resistance

These measurements aimed to determine the condition of insulation between the windings to ground or between two windings. A common method is to provide a dc voltage and represents the condition of the unit megohm insulation. Measured insulation resistance is a function of leakage current that penetrates through the insulation or through the leak in the external surface. Insulation resistance testing may be influenced temperature, humidity and leakage paths on the external surfaces such as dirt on the bushing or insulator. Megaohm meters typically have a capacity of testing 500, 1000 or 2500 V dc.

Mega Ohm meter gauge

Polarization Index Test

The purpose of the polarization index test is to ensure the equipment is operated or even feasible to do overvoltage test. Polarization index is the ratio of insulation resistance at the 10th minute by minute to 1 with a constant voltage.

Total current that arise when providing a dc voltage steady state consists of: 

1. Charging current due to the nature of the measured capacitance of the insulation. This flows down from the maximum value to zero very quickly.
2. Absorption of molecular charge current due to shifting in isolation. These transient currents disappear to zero more slowly
3. leakage current is a conduction current in insulating concrete. Leakage current varies depending on the test voltage. Also includes a leakage current due to leakage at the surface due to contamination.

 

Description:
R = insulation resistance (MΩ)

C = 1.5 for oil filled transformers at a temperature of 20o C 30.0 to untanked oil-impregnated transformers

E =  voltage rating (V) on the inter-phase delta connection, the phase neutral star connection

kVA = rating capacity of the tested winding.

Insulation condition based on the index of polarization

Power Transformers Part

Power Transformers Part

1. Refrigerant

The temperature at which the transformer is operating will be influenced by the quality of the network voltage, losses in the transformer itself and the ambient temperature. High operating temperatures will cause damage to the transformer insulation paper. Therefore, effective cooling is needed.

Transformer insulating oil in addition to the isolation medium also serves as a coolant. At the time of the oil circulates, the heat emanating from the entanglement will be brought by the oil circulation on track and will be cooled on the fin - fin radiator. The cooling process can be aided by a fan and circulation pump in order to improve cooling efficiency.

Power Transformers Radiator

2. Oil preservation & expansion (Conservator)

When the operating temperature rise in transformer oil, insulation will expand so that its volume increases. Conversely when the operating temperature decreases, then the oil will shrink and the volume of oil will go down. Conservators used to accommodate the transformer oil at mengalamui temperature rise.

Power Transformers Conservator 

Silica gel

To avoid the transformer oil does not deal directly with outside air, then the current conservator designed using brether bag / rubber bag, which is a kind of rubber balloon that is placed inside the conservator tank.

POWER TRANSFORMERS

1. Definition and function POWER TRANSFORMERS

The transformer is an electrical device which serves to channel the power / energy from high voltage to low voltage or vice versa.The transformer uses the principle of faraday law of induction andthe Lorentz law in distributing power, where the alternating currentthat flows around an iron core iron core then it will turn into a magnet. And if the magnet is surrounded by a winding it on both ends of the windings is the potential difference will occur (Figure1.1).

Figure 1.1. Alternating current around an iron core

Current flowing in the primary winding will induce iron coretransformer that will flow in the iron core and the magnetic flux of this magnetic flux will induce a secondary winding secondarywinding so that at the end there