This article discusses typical transformer configurations and measurement sequences as specified in the IEC 60076-18 international standard. Although it is based on the operation of the N4L SFRA45 sweep frequency response analyser from Newtons4th, the article provides a baseline for measurements which can be applied in most situations.
Additional measurements may be carried out either to provide extra information or to match previous measurement configurations or test sequences.
Using the default settings of the SFRA45, the instrument sweep is already set to a 10 Hz to 20 MHz sweep in 2000 steps. This can be altered in the “SWEEP” menu, in most cases 10 Hz to 2 MHz will be an acceptable frequency range for the test.
Fig. 1: The leads should be connected as shown when conducting the
0 dB check (Newtons4th).
When using the N4L clamp set one should enter the CH1 menu and select “N4L Clamps – YES”. All that is now required is to turn on the generator and select “START” to begin the sweep.
It should be observed that the bode plot will provide an equal to or better response than ±0,5 dB @ 2 MHz and better than ±3 dB at 20 MHz.
IEC 60076 states that the maximum attenuation during a 0 dB check should be <0,6dB at 2 MHz.
Additional measurements, such as capacitive inter-winding, inductive inter-winding and end-to-end short-circuit tests can also be carried out.
Other windings with a fixed amount of turns will be tested in a tap position of maximum number of turns in the test circuit.
This measurement approach is intended to provide measurements both with and without the tap winding in circuit; this aids the diagnosis of a fault.
For neutral or changeover positions, the direction of the “change” of the tap-changer itself will be in the “Voltage-Lowering” direction, the direction of movement should always be recorded.
With a de-energised tap changer, the standard measurement will be on the “as-found” DETC tap position. In the factory, the standard measurements will be on the tap with the highest number of turns in circuit and in the service position, if known.
| Test No. | Signal and reference (CH1) | Response (CH2) | Tap position |
| 1 | Neutral | Line terminal phase 1 | Max turns |
| 2 | Neutral | Line terminal phase 2 | Max turns |
| 3 | Neutral | Line terminal phase 3 | Max turns |
| 4 | Neutral | Line terminal phase 1 | Tap winding out of circuit |
| 5 | Neutral | Line terminal phase 2 | Tap winding out of circuit |
| 6 | Neutral | Line terminal phase 3 | Tap winding out of circuit |
If the service position is not known, then the measurements will be on each DETC position unless data is available from a previously recorded identical transformer.
Star and auto connected windings with a neutral terminal
As a rule, the signal (generator) should be applied to the neutral terminal, or for series windings the lower voltage terminal.
If in previous tests the measurements were performed with the generator connected to the line terminals, this test should also be carried out.
A star connected winding with no neutral
Star connected windings with no neutral brought out can be treated as a delta winding.
Delta windings and other windings without an accessible neutral
If delta windings can be split, the measurements should be carried out as such. As a rule, measurements should be made on each phase, injecting the reference (generator) signal to the terminal with the lowest number or letter nearest the start of the alphabet.
For example:
A to B, V to W or 1 to 2
| Test | Signal and Reference (CH1) | Response (CH2) |
| 1 | A, U, R or 1 | B, V, S or 2 |
| 2 | B, V, S or 2 | C, W, T or 3 |
| 3 | C, W, T or 3 | A, U, R or 1 |
For delta tertiary or stabilising windings, the delta shall be closed.
If the delta tertiary is earthed at one corner during service, this should be removed if possible without the removal of liquid or gas.
Zig-zag connected windings
Zig-zag connected windings can be treated as a star winding with a neutral connection.
Two-winding three-phase transformers
One measurement of each phase on every winding, this would be a total of six measurements for a transformer without taps and nine with an OLTC.
Three phase auto-transformers
Standard measurements consist of one on each phase of the series winding and the common winding, with additional measurements for common windings with an OLTC. A total of six measurements for transformers without tap changers and nine for transformers with a tap changer.
If a tertiary winding is brought out, three additional tests on this winding will be required.
| Name of function | Description |
| Identifier | Typically the customer serial number or the location number of the transformer/reactor |
| Date | Date the test was carried out |
| Time | Time the test was completed |
| Manufacturer | Manufacturer of the transformer |
| Serial No | Serial number of the transformer |
| Measuring equipment | Device serial and model numbers |
| Peak voltage | Vpeak setting in the output menu |
| Reference terminal | The identifier of the transformer/reactor terminal to which the generator/CH1 connection was made |
| Response terminal | The identifier of the transformer/reactor terminal to which the CH2 connection was made |
| Connected terminals | The identifiers of the transformers/reactors terminals that were connected together during the test in the format ID1-ID2-ID3, ID4-ID5-ID6 (e.g. A-B-C , D-E-F would denote terminals A, B and C were connected together and terminals D, E and F were separately connected together) |
| Earthed terminals | The identifier for each terminal connected to the transformer/reactor tank during the test |
| OLTC | Tap position indicated on the transformer/reactor during the test |
| Previous OLTC position | The tap position from which the tap changer was moved in order to be at its current position |
| DETC position | Position of the DETC |
| Temperature | Transformer/reactor temperature |
| Fluid filled | Yes/No |
| Comments | Provides additional comments box. Example “service” if the bus bars are removed but bushings installed. |
| Unshielded length | Length of earth braid used |
Phase shifting transformers
Standard measurements are from the input to the output terminal on each phase and from the neutral of the shunt winding to the output on each phase. This should be performed on the neutral tap and the extreme tap, being a total of
18 measurements.
If the transformer is a “two-core” type with external interconnections that can be removed on site then the transformer can be treated as two separate entities.
Reactors
Standard measurements are from the input to the output terminal on each phase, a total of three measurements. Shunt reactors can be treated as a star wound transformer, with three measurements without taps and six measurements with taps.
Additional tests
Any additional measurements should be specified by stating the signal/reference, response, earth arrangements and if the terminals are floating or connected together. The current tap position and the previous tap position for each additional measurement should be recorded.
Saving data
IEC60076-18 states that the instrument must record and save data in a defined XML format as per Table 3, for clarity we have defined each item. Once a sweep is complete the engineer can save the sweep data to the desired memory location.
Contact Jacques Franken, AISA, Tel 011 403-2247, sales@aisa.co.za
