TRAX Multifunction transformer and substation test system

The TRAX test set is a well-equipped electrical laboratory in a box, complete with an integral oscilloscope. Fundamentally, TRAX test sets generate and monitor AC and DC voltages and currents. These voltages and currents can be used as the basis for virtually any test procedure, which means the future extendibility of TRAX is potentially unlimited. New tests can be performed manually by experienced users, or new apps can be developed to automate them. Accessories, such as the three-phase switch box, can also be added.

Megger has a universally acknowledged commitment to safety and as such has equipped TRAX with a full range of safety features. These include ground-loop detection, dual interlocks, and facilities for fast discharge of inductive measurement objects. Sequence monitoring is also provided to ensure that all test connections are made correctly and in the right order. There’s also a readily accessible emergency shutdown button, and provision for connecting an optional strobe warning light.

FRSL stands for frequency response of stray losses. It’s a technique for detecting strand-to-strand short-circuits in transformer windings by performing short-circuit tests over a wide range of frequencies. Diagnostics based on FRSL rely on comparing the results obtained in a test with earlier measurements, with tests carried out on an identical transformer, or between phases. Measurements are made on the high voltage (HV) side of the transformer, with the low voltage (LV) side short circuited. FRSL tests can be performed with Megger FRAX and TRAX test sets.

The measuring techniques used are similar but, as the name implies, narrowband DFR uses a much more restricted range of frequencies – usually from around 1 Hz to 500 Hz. Also, the results are analysed directly rather than by using modelling techniques. It takes much less time to carry out a narrowband DFR test than a full DFR test – around two minutes compared with upwards of twenty minutes in some cases  – but the narrowband test doesn’t provide the estimated moisture content for the cellulose insulation or oil conductivity. What it does do is provide an earlier indication of problems than a traditional power factor/tan delta test performed only at power frequency. It also confirms that seemingly good power factor/tan delta values really are good and allows the transformer’s individual temperature correction (ITC) factor to be determined. Narrowband DFR testing is supported by Megger test sets in the TRAX and DELTA4000 ranges.

The TRAX measures dynamic resistance in the on-load tap-changer by simultaneously measuring the test current together with voltages on both HV and LV windings as the OLTC operates. These results are used together with transformer modelling. The LV winding is left open. Due to the consequent inductance in the circuit, the change in voltage measured across the HV winding is rather large. This voltage is a sum of inductive and resistive voltages and cannot be used for directly calculating the resistance in the circuit. However, the voltage measured across the LV winding is purely inductive. By using transformer model parameters to calculate the inductive voltage on the HV winding, this value can be deducted from the total measured HV winding voltage and then the resistance in the circuit can be calculated. This is a Megger patented method.

With DC resistance measurements, the objective is always to try to saturate the transformer core as this reduces the effective inductance of the winding and allows the test current to stabilise more quickly. Saturation requires a minimum test current of 1 % of the rated current for the winding. It is, however, usually advantageous to use a somewhat higher test current than this, particularly when the rated current of the winding is high, in which case, a higher test current may speed up the time to saturation. If the test current is too low, it will often be found that successive measurements give inconsistent results. Nevertheless, test currents of more than 15 % of the rated current should be avoided, as they are likely to lead to erroneous results due to the heating of the winding. In most cases, the optimum test current is between 1 % and 15 % of the rated current. It should be noted, however, that it is the compliance voltage of the test instrument that determines the saturation rate of a transformer during this test. For this reason, compliance voltages above 40 V DC are preferred. The Megger TRAX provides up to 100 A true DC at up to 50 V compliance voltage.

Winding resistance tests are usually carried out on new transformers after they are delivered to site and before they are put into service as a way of detecting transit damage. Such tests will also provide baseline results for comparison with measurements made throughout the life of the transformer. Winding resistance tests should be performed as part of scheduled maintenance as an aid to detecting incipient faults; this is arguably one of the most important routine tests that one can do on a transformer. Finally, winding resistance tests are invaluable when finding faults in transformers, as many transformer faults or problems will cause a change in the DC winding resistance.

No, but some people refer to it as such. A ‘ripple test’ does constitute a dynamic measurement on an OLTC. In all OLTC dynamic measurements, a current is injected in the tap changer, either in one phase or all phases, and during the operation of the tap changer, the current and/or the voltage is measured as a function of time. In a ‘ripple test’, the current is measured and the result is presented in a current-time diagram or as a percentage ripple value. Ripple is the magnitude by which the test current decreases during the tap change; it is expressed as a percentage of the test current. The behaviour of the current is affected by the resistance of the transition path and the speed of the tap change operation. However, a ripple test cannot provide transition resistor values or transition times.

The use of an instrument that has been designed with transformer resistance testing in mind is strongly recommended as it will give more dependable results more quickly and more safely, especially with large transformers. It will also include provision for demagnetising the transformer core after testing has been completed. This is important as, if a transformer is returned to service with a magnetised core, a large and potentially damaging inrush current may flow. It may not, however, be necessary to use a dedicated transformer ohmmeter. The TRAX multifunction tester provides an extensive range of tests for transformers, including not only winding resistance tests, but also (for example) turns ratio tests, leakage reactance measurements, and power factor/tan delta measurements. These testers can also perform basic measurements on circuit breakers, protection relays, and many other items of equipment used in power distribution networks. For many users, they are a better and more useful investment than a dedicated single-function transformer ohmmeter.