OMICRON Magazine

Series unit measurements Similar to measuring the voltage ratio at different tap posi- tions of a common network transformer, it is important to verify the specified phase shifting range between the line terminals of the “S” and “L” sides of PST. We used our portable 3-phase transformer test system, TESTRANO 600, to perform a simultaneous measurement of the phase shift, voltage turns ratio and exciting current on all three phases. The results con- firmed that the operating range was between +10° and –10° with a step width of 0.87° under no-load condition. A big challenge for testing engineers, especially during com- missioning testing on site, is determining whether the PST is operating in an advanced or retarded position. Engineers need to know if the PST is enhancing or blocking the power flow in the respective branch of the network. Making this determination is required in order to define the source and load side terminals. By using a 3-phase measurement, the phase relation of the source and load side voltages and currents can conveniently be displayed in a vector diagram to determine the state of the control. An example of the results obtained at different tap positions as shown in Figure 2. The vector interpretation of the measurement results of the phase shift control direction gives us an unambiguous indication of which of the voltage vectors on the “S” or “L” side is ahead or delayed. An additional oscilloscope record- ing that would require additional equipment and testing time is no longer necessary thanks to this visualization of the measurement results. Figure 3: Measurement results of the voltage converter of the excitation unit as a function of the OLTC position Figure 2: The results of phase shift measurements “S” – “L” for three characteristic positions Phase shift “S”–“L” regulation from –10° to 0° Orientation of voltage vector ∆U Phase shift “S”–“L” regulation from 0° to +10° Orientation of voltage vector ∆U phase shift 10° phase shift 0° phase shift –10° uA “S” uB “S” uC “S” uA “L” uB “L” uC “L” 90° 90° 90° 0° 0° 0° 180° 270° 270° 270° uA “S” uA “S” uC “S” uC “L” uB “L” uB “S” uC “S” uA “L” uA “L” uB “L” uB “L” uB “S” uA “L” uA “S” uC “L” uC “S” uB “S” uC “L” 20 0 2 4 6 8 10 12 40 60 Tap changer position U [V] 20 0 15 17 19 21 23 25 40 60 Tap changer position U [V] 32