Recognizing if a Tap Changer Has Multiple Nominal Positions

Transcription

1 Application Note Recognizing if a Tap Changer Has Multiple Nominal Positions The middle position of a tap changer is usually called a nominal or neutral position. Normally, transformer nominal voltage is at neutral/nominal position and regulation adds or subtracts voltage as required per user s specification. Operation of a tap changer around neutral position may be peculiar. Various arrangements exist, where a neutral position is just one single position on the tap changer, or three positions are assigned to the neutral. As two different operations may characterize the neutral position: 1. operation of a tap changer selector from tap to tap, and 2. operation of a reversal switch (or changeover switch) changing from PLUS to MINUS, or Fine to Coarse, tap changer manufacturers construct this transition as one, or three positions. If constructed with three positions, MR for example gives name to these positions as A, B, C for example: 11A, 11B, 11C as shown in the Fig.1. Figure 1. MR dial on a tap changer 1

2 Where ABB uses only letters A and B for example 11A, 11, 11B. Figure 2. ABB dial on a tap changer One important aspect of operation through neutral position is whether the motor stops or not at those additional positions. If the motor does not stop, it operates for twice the length of one operation. There, the DVtest record needs to extend recording time to catch the second transition, as shown in the Fig 3 below. Figure 3. DVtest record around neutral position 2

3 For this type of motor operation, through double neutral positions, the DV-Win option has been developed extending the record before and one record after the neutral position by 7 seconds. In this way the extended record is now 17 seconds. As another option to these multiple neutral positions, tap changers that operate through all three positions without motor stopping have been tested. This is visible on the graph below. Figure 4. DVtest showing 3 +1 motor operation For all different tap operations, except for normal 2+2, it is recommended to use a motor trigger recording. This type of triggering keeps recording as long as motor is working and data cannot be lost. Figure 5. DVtest showing 2+2 motor operation 3

4 USA Tap Changers In the USA predominant numbering of tap positions with 33 taps is from 16L through N to 16R (or 16Lower through Neutral to 16Raise). The tap changer manufacturers also provide either one single N position or three of them. In case of three positions they are usually numbered as LN, N, RN. The voltage at these three positions is the same usually the nominal transformer voltage. Testing Sometimes, the tap changer motor may not be powered by electricity, as in case of a spare unit. A crank-shaft exists that allows a user/tester to operate the tap changer manually. If the tap changer is operated manually, there is no concern the operation would go through 2 or 3 neutral positions without stopping. Manual cranking would stop after each tap position is reached. Selecting the DV-Win Parameters When selecting parameters to enter on a DV-Win display for computer-operated DVtest, three options are available in a Standard Tap Changer Settings selection: - Multiple neutral positions: YES/NO - Nominal position type : a,b,c / a,b - Drive: Motor/manual These values should be selected based on the above explained operational parameters. 4

5 Figure 6. DV-Win selection If the tap changer has only one nominal position, like the one in the Figure 7, a selection of the Multiple nominal positions in DV-Win should be NO. This would automatically deselect the other two options as they are not relevant. Figure 7. Tap changer dial without multiple neutral positions 5