Matz Ohlen - Director - transformer test systems
The dynamic resistance measurement (DRM) technique for circuit breaker diagnostics was invented by Megger and is used extensively on high voltage circuit breakers to determine the condition and deterioration of arcing contacts inside the breaker. DRM is also an advanced diagnostic tool for load tap-changers to detect problems related to transition times and resistance values.
A newly introduced accessory power unit for use with Megger circuit breaker analysers (CBA) provides three 1 A or one 5 A constant-current outputs, as well as facilities for safely discharging the transformer windings via the current and voltage leads. In conjunction with a circuit breaker analyser, this power supply forms a complete and convenient set up for dynamic voltage and dynamic resistance testing on load-tap changers.
Figure 1 – DRM test on a load-tap changer in good condition, using a constant current source. Blue – voltage/resistance. Red – test current. H1 1-2 tap change with LV shorted.
The example in Figure 1 shows resistance change/transition time results produced using this equipment on one tap changer of a 20 MVA transformer. The transition times and resistor values can be readily determined from these results. A new test method/application – dynamic voltage measurement – has recently been added for performing dynamic measurements on load tap-changers. Dynamic voltage measurement can be used for verification of continuity and contact/resistor switching times. Depending on the settings used and test current it can also give an indication of diverter resistor values.
Figure 2 - Set up for dynamic voltage measurement.
Dynamic voltage measurements are preferably done on the opposite side of the transformer to the load tap changer – that is, on the LV side if the load-tap changer is on the HV side – recording the inductive/induced voltage. A typical arrangement is shown in Figure 2.
The main advantage of this method is its simplicity. It is only necessary to connect a voltage measurement input to one (or three) windings, inject the test current and record LV voltage during the tap transition. With some circuit breaker analysers, such as the Megger Egil, no computer is needed – the voltage versus time curves can be recorded and printed without additional equipment.
To perform the test, the power supply is connected to inject about 0.1-1% of rated current for the winding (typically 1or 5A). The voltage on the secondary is measured during the tap transition. The measured voltage will reflect the inductive voltage generated inside the tap-changer when the diverter circuitry is switched. Measurements can be performed one tap at a time or over the complete operation over all taps.
An example of a dynamic voltage measurement for one tap transition is shown in Figure 3. As will be seen, the individual, parallel and total resistor transition times can easily be identified.
Figure 3 Measured dynamic voltage on the LV side of a transformer during a tap transition. (Tap changer on HV side). Total resistor transition time 49 ms. 6 MVA transformer, Dyn11, 21/11 kV, test current 4 A.