GE D30 Instruction Manual

Also see for D30 series: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

CHAPTER 9: THEORY OF OPERATION SERIES COMPENSATED LINESD30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL 9-199Distance protection elements of the D30 deal with the problem of current inversion by using a multi-input-comparatorapproach as described in the Distance Characteristics section. If the current inversion happens, the distance elements aresecure on reverse faults because multiple conditions involving fault-loop, negative-sequence and zero-sequence currents,and the memory voltage are checked prior to declaring a forward fault.On close-in forward faults beyond the series capacitors as seen from the relaying point, the current inversion phenomenoncan take place for a short period of time. The condition cannot sustain for a long time as very high fault currents wouldoccur, causing large voltage drops across the series capacitors and prompting the overvoltage protection of thecapacitors to operate quickly. This effectively removes the series compensation and eliminates the current inversion.However, when the currents used by distance comparator (fault-loop current for ground and phase distance protection,and the negative- and zero-sequence currents for ground elements) stay shifted by more than 90 degrees from theirnatural fault position determined by the user as the element characteristic angle, the distance elements can fail to pick upon such a forward fault for the brief period of current inversion. This is an inherent attribute of the 100% memory polarizedmho element, and not a weakness particular to the D30 relay.Therefore, for dependability, it is recommended to use high-set phase overcurrent protection for direct tripping on close-infaults potentially causing current inversion, and overreaching ground fault directional overcurrent functions (such asnegative-sequence, ground, or neutral) for communication-aided schemes.The problem of steady-state overreaching due to the negative reactance of the series capacitors can be addressed in theD30 in a traditional way by shortening the reach of an underreaching distance elements to the net inductive reactance ofthe line between the potential source and the far end busbar(s). This generic approach has two major drawbacks. First, itleaves large portion of the line uncovered by the directly tripping distance protection. Second, it does not solve thetransient overreaching problem caused by sub-synchronous oscillations.Therefore, the D30 offers a unique option for dynamic reach control that is effectively based on the magnitude of thecurrent flowing through the series capacitor bank(s). The underreaching distance functions can be set as for plainuncompensated line, that is, using the impedance of the line alone, and the relay then controls an effective reachaccordingly using the current magnitude as illustrated in the following figure.The reach is reduced sufficiently to cope with both steady-state and transient overreach phenomena. For large degrees ofcompensation and small-current faults, the transient overreach can be as high as 100%. This means that fast distanceprotection is not achievable. The adaptive D30's mechanism guarantees security on external faults. Use overreachingground fault directional overcurrent functions (such as negative-sequence, ground, or neutral) for dependability.Figure 9-7: Dynamic reach controlSection (a) of the following figure shows the effect of adaptive reach control for low-current external fault. The reach isreduced sufficiently to cope with both transient and steady-state overreach. Section (b) shows a high-current externalfault. The air gaps or MOVs conduct the majority of the fault current and neither steady-state nor transient overreach