In the normal operation of machinery, the insulation is subjected to moisture, oil, dust, electrostatic stress due to machine operation and a host of other elements. Hence, insulation ages and deteriorates. It is vital that the health of the insulation be monitored continually to avoid sudden, catastrophic failure of machines.
Principle of Insulation Resistance Measurement
The method used to measure insulation resistance is based on Ohm’s law. A high voltage is applied across the resistance; the current that flows through the insulation is measured. The ratio of voltage and current gives the resistance. The value of the insulation resistance is usually in the order of mega ohms
Instruments used in Measurement
The instrument used to measure Insulation Resistance is known as the Megger. It is similar in principle to the ohmmeter except for the fact that a higher voltage is used. The typical meggars have a test voltage of 500V, 2500V or 5000V. The Meggar has a high internal resistance hence, there it is safe to use despite the high voltage generated. The meggar has 3 terminals. Line, Earth and Guard.
The test voltage appears on the “Line” Terminal. This terminal is connected to the winding whose insulation needs to be checked. The “Earth” Terminal is connected to the ground. The “Guard” Terminal is connected to the surface of the insulation to measure the surface currents which tends to flow along the surface of the insulation.
Method of Insulation Resistance Measurement
The winding to be tested should first be isolated. The other windings of the machine which are not being tested should be connected to the ground. The voltage is applied to the winding and the reading is taken after about 60 seconds. The reading is noted. After the test is over, the winding needs to be “discharged”. This is because the insulation acts as a dielectric forming a capacitor between the winding and the earth. This can store charge and can deliver a shock if not discharged. Discharging can be done by connecting to the ground.
What should be the value of the Insulation Resistance?
The Insulation Resistance thus measured is usually in the order of mega ohms. A general rule of the thumb is that the minimum value should be greater than 1 mega ohm for every 1kV rating of the machine. Thus, for a machine rated for 11kV, the minimum acceptable value would be 11 mega ohms. Temperature has a direct impact on the value of the Insulation Resistance. The Insulation Resistance decreases with increase in temperature. Thus the values should be normalized for a standard temperature.
That is, a value measured at 20 deg. C cannot be compared with a value measured at 30 deg. C. The value at 30 deg. C needs to be corrected. A general rule of thumb is that the insulation resistance decreases by a factor of 2 for every 10 degree rise in temperature.
Hence, the value taken at 30 deg. C needs to be multiplied by a 2 to get a value corrected to 20 degrees.
How do we ensure a good value of IR?
The Insulation Resistance of a machine depends chiefly on the dryness of the windings. The entry of moisture into the windings lowers the Insulation Resistance. The ingress of moisture can be prevented by ensuring that the windings are kept dry. Special heaters known as anti-condensation heaters are provided in machines to keep them dry. It must be ensured that these heaters are kept on.
How do we improve the Insulation Resistance value?
If machines are found with low Insulation Resistance values below the permissible limits, heating the windings by connecting lamps around them is an effective method of driving moisture from the windings. If no improvement is seen even after heating, other reasons such as insulation wear or deterioration can be suspected.
Other parameters related to the health of the Insulation are the Polarization Index(PI), tan delta, hipot test, step test, etc
Tan Delta Testing
Earth Resistance Measurement