3.4.27 SURG - Surge Stress Test
UNDER NO CIRCUMSTANCES SHOULD THE SURGE BE APPLIED TO THE INPUT WINDING OF A STEP UP TRANSFORMER SUCH THAT ITS OUTPUT IS BIGGER THAN 7KV
The purpose of this test is to detect an inter-turn short by applying a number of high voltage impulses (or surges) to the selected winding.
Each impulse produces a sinusoidal transient which eventually decays to zero.
During the test, the tester measures the area (in Volt - seconds) under each transient. The decay is much shorter in a faulty winding, and is detected as a smaller value for the measured area.
On selecting ‘SURG Stress Test - Surge’ from the Available Tests window, the following form-fill dialogue box will be displayed:
To program the test :
- Enter the desired test voltage.
- Enter either a nominal winding inductance or select AUTO mode.
When a fixed inductance is entered, the AT3600 uses this value to estimate the frequency and duration of the transient waveform and thus determine the digital waveform sampling rate that is used.
In AUTO mode, the tester automatically determines the ideal sample rate and duration after testing the first sample in a batch. AUTO mode is recommended for new test programs. If you are converting an existing test program from fixed to AUTO surge testing mode, beware that under some conditions the measured result may change.
Note that the Inductance value may also be entered from a measurement on the specimen by using the Measure button.
- Enter the number of impulses.
You may choose only to have one impulse; the test will be quicker to execute, but there will be less chance of promoting a failure in a transformer where the inter-turn insulation is damaged and weakened, but not yet causing a complete short. More impulses give a greater chance of detecting damaged insulation.
- Enter the high and low terminal names. Use the mouse to pull down the menu along side the fill-in box, and double click to select the terminal.
- Select (mouse click the button) the type of limits you require and enter the values.
It is very difficult to predict the minimum and maximum areas from theory. A practical approach is to use the Measure Button (see section 3.9 of this chapter):
||Enter a nominal value and then the limits as negative and positive
||Enter minimum and maximum values.
||Enter just a minimum value.
Measure the area on a known good transformer; let this result be area AG.
Wrap an additional single turn round the core, short the two ends together, and re-measure the area; let this result be area AF.
Set the limits as follows:
MaxArea = 3AG / 2
MinArea = (AG + AF)/2
Remember that these limits are taken from only one transformer, and may need to be revised after more have been tested.
- Select OK. The test specification will then be displayed in the program window.