To avoid damage due to overheating and intense

  • No School
  • AA 1
  • 19

This preview shows page 9 - 10 out of 19 pages.

To avoid damage due to overheating and intense partial discharges of the insulation during the high voltage AC tests international practice is to subject the windings to DC over voltage tests. A variety of direct voltage (DC) testing techniques have been devised to monitor and assess the condition of stator winding insulation systems. These methods range from the simple withstand or proof tests to more refined rampled voltage test in which the DC high voltage is applied as a linearly increasing ramp function and the current response is recorded. 3.1.2.1 Evaluation of deterioration by Hipot method: The dc hipot tests are performed by several methods. DC test sets being of lower capacity are portable and power consumption is much less. The ratio between DC and AC tests varies between 1.2 and 2.5. Often a value of 1.6 is chosen usually for the machines in operation. The disadvantage to DC testing is that actual service conditions are not simulated during this test as stress distribution is governed by resistivity and the insulation is unevenly stressed. On the basis of extensive tests Rushall and Simons [34] have also concluded that the high voltage DC testing method has no valid basis for non-destructivity indicating the service-ability of the insulation of HV machines. IEEE standard 95 [35] gives the guidelines for conducting DC tests and 2002 version highlights a new variation of the DC hipot test called DC Ramp test. (a) Conventional dc hipot In this method a suitable high voltage is applied quickly to stator winding terminal and held for either 1 or 5 minutes and after this time the voltage is gradually reduced to zero. If the insulation is sound, there will be no high current surge and power circuit breaker will not trip. If the power circuit breaker trips then it is likely a puncture of insulation has occurred. (b) Step stress Hipot In this method the applied DC voltage is increased in 1 kV steps, with each voltage level being held for I minute before it is increased again. The DC current is measured after the end of each step and current plots are obtained with voltage. The trend is generally a line with a gentle upward curve. An abrupt increase in current is an indication of weakened insulation. (c ) DC Ramp Hipot In this method, the DC voltage is smoothly and linearly increased at a constant rate, usually 1 or 2 kV/minute and there are no discrete steps in voltage or current. The current vs voltage plot is automatically graphed and displayed. The advantage of this method is that it is by far the most sensitive way to detect when a current instability is occurring, since the capacitive charging current is not changing with time. However the disadvantage is that it does not duplicate the voltage stresses applied across the insulation when it is in operation [36].
Image of page 9
Image of page 10

You've reached the end of your free preview.

Want to read all 19 pages?

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask You can ask (will expire )
Answers in as fast as 15 minutes