TTRU3 True three-phase transformer turns ratio tester

• Check that the power cord is fully inserted into the TTRU3.
• Check that the power source is outputting voltage at acceptable levels and frequency.
• Check that the power cord is fully inserted into the source.
• Check that the power switch is in the correct position ( I ).
• Set the power switch to off ( O ). Wait 30 seconds. Set the power switch to on ( I ).
• Try another power cord.

• Check lead connections.
• Reference the nameplate to ensure leads are connected to the correct bushing.

Check the OLTC wiring diagram and ensure leads are connected to correct terminals.

• Contact your IT department for primary assistance when connecting any device to your PC
• Check the USB cable is fully inserted into the TTRU3
• Check the USB cable is fully inserted into the PC
• Check the TTRU3 is powered on
• Check the TTRU3 software is installed
• Check the TTRU3 software is not running in ‘simulation’ mode
• Check the TTRU3 is running
• Move the USB cable to another USB port on your PC
• Try another USB Cable
• Try another PC

• Check the battery is inserted into the printer
• Charge the printer battery using the supplied charger
• Check the printer paper is inserted properly
• Check the USB cable is plugged into the printer
• Check the USB cable is plugged into the TTRU3 USB port
• Check the printer is turned on by holding down the power button
• Try other USB ports

The ratio is the measured transformer turns ratio (TTR), calculated using both the voltage applied to one side of the transformer and the induced voltage measured on the other. Calculated TTR is determined from the transformer's nameplate voltages and the k factor, if necessary, as given in the table below. With the measured TTR in hand, a percentage deviation from the calculated TTR can be computed, either manually or automatically by the TTRU3. As per IEEE, the percentage deviation between measured and calculated TTR should be within a ±0.5 % tolerance.

IEEE documents cases of transformers that have a load tap changer in their low voltage side with an overall low number of turns that will cause some of the tap steps not having the same number of turns as others. Thus, the variation per tap is not uniform and might be outside the 0.5 % tolerance of deviation from nameplate values. In these cases, there are two criteria used to evaluate the results. First, the measured TTR at both extreme ends of the tap changer (highest and lowest) should be within the 0.5 % tolerance from the calculated TTR. Second, for any given tap, all three phases of the transformer should have the same voltage ratios.

The excitation current test is a routine measurement that can be used to detect major problems in the magnetic core structure and winding defects, like shorted turns. An excitation current  measurement is often performed as a standalone test using a power factor test set, as it is normally made at rated frequency and voltages up to 10 kV. The results are voltage dependent but, due to the fact that the measurement’s evaluation relies heavily on pattern recognition, the numbers obtained during TTR testing – even when performed at considerably lower voltages – can be used as a good tool to diagnose the issues mentioned above, especially when having previous data from tests performed at the same voltage. A typical “phase pattern” presented by the excitation current test results obtained for all phases at a given tap position of a 3-phase transformer is H-L-H. The excitation current measured for the two outer wound phases should be of similar magnitude while the excitation current of the centre wound phase is the lowest in magnitude.

The phase angle deviation, displayed in either degrees (minutes) or radians, is the phase relationship between the voltage signal applied to the high (or low) winding and the voltage signal measured at the low (or high) voltage winding. The phase deviation together with ratio error can be used as a low cost method of verifying accuracy class of all types of PTs and CTs at ‘zero burden’. The phase deviation between the high and low side of a transformer is generally very small. If there is deterioration or damage in the transformer core, however, the phase deviation can change significantly. Building a transformer core with high permeability, low loss material and with no defects between laminations – in other words, no shorts between adjacent layers in the core – will help minimise the eddy currents and thus reduce the phase deviation. One can therefore state that any significant phase deviation reflects a core which is not efficient. If a transformer exhibits higher losses than expected, the core is the probable cause and phase deviation a visible result.