GE Multilin D30 Line Distance Relay 9-79 APPLICATION OF SETTINGS 9.4 PHASE DISTANCE THROUGH POWER TRANSFORMERS99.4PHASE DISTANCE THROUGH POWER TRANSFORMERS 9.4.1 PHASE DISTANCE PROTECTIONa) DESCRIPTIONPhase distance elements of the D30 could be set to respond to faults beyond any three-phase power transformer. The relayguarantees accurate reach and targeting for any phase fault. Moreover, the current and voltage transformers may belocated independently on different sides of the transformer.The following setting rules apply to this feature:1. A given distance zone is terminated by location of the VTs, not the CTs.2. Consequently, the positive-sequence impedance of a transformer must be included in the reach setting only if thetransformer is located between the potential source and the intended reach point.3. The current signals require compensation if the transformer is located between the CTs and the intended reach point.If this is the case the CT connection setting shall be set to transformer connection and vector group as seen from theCTs toward the reach point. Otherwise, the CT connection setting shall be set to "None".4. The voltage signals require compensation if the transformer is located between the VTs and the intended reach point.If this is the case the VT connection setting shall be set to transformer connection and vector group as seen from theVTs toward the reach point. Otherwise, the VT connection setting shall be set to "None".5. The reach setting is entered in secondary ohms and as such must take into account location and ratios of VTs and CTsas well as voltage ratio of the involved power transformer.The following equations explain the setting rules. Consider two applications as shown in the figure below:Figure 9–3: PHASE DISTANCE PROTECTION LOOKING THROUGH A POWER TRANSFORMERwhere: ZX = intended reach impedance for Zone 1 (primary ohms)ZH = intended reach impedance for Zone 3 (primary ohms)ZT = positive-sequence impedance of the transformerVX, VH = transformer rated voltagesnCT = transformation ratio of the CTsnVT = transformation ratio of the VTsZ1: Z1 reach setting (secondary ohms)Z3: Zone 3 reach setting (secondary ohms)830718A1.CDRXHZ XZ1ZH Z TZ3(a)XHZ XZ1ZH Z TZ3(b)
