CHAPTER 6: ACTUAL VALUES METERINGL90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 6-156DELTA-connected instrument transformersThe zero-sequence voltage is not measurable under the Delta connection of instrument transformers and is defaulted tozero. The table below shows an example of symmetrical components calculations for the ABC phase rotation.Table 6-1: Symmetrical components calculation example* The power system voltages are phase-referenced – for simplicity – to VAG and VAB , respectively. This, however, is arelative matter. It is important to remember that the L90 displays are always referenced as specified under SETTINGS SYSTEM SETUP POWER SYSTEM FREQUENCY AND PHASE REFERENCE.The example above is illustrated in the following figure.Figure 6-4: Measurement convention for symmetrical components• ABC phase rotation: • ACB phase rotation:SYSTEM VOLTAGES, sec. V * VT conn. relay INPUTS, sec. V SYMM. COMP, sec. VVAG VBG VCG VAB VBC VCA F5ac F6ac F7ac V0 V1 V213.9∠0°76.2∠–125°79.7∠–250°84.9∠–313°138.3∠–97°85.4∠–241°WYE 13.9∠0°76.2∠–125°79.7∠–250°19.5∠–192°56.5∠–7°23.3∠–187°UNKNOWN (only V1 and V2can be determined)84.9∠0°138.3∠–144°85.4∠–288°DELTA 84.9∠0°138.3∠–144°85.4∠–288°N/A 56.5∠–54°23.3∠–234°
