Causes of CT Saturation From Transient DC Offset

When a fault current includes a DC component, a sudden disappearance of that component can indicate CT saturation. Previous studies and testing (see [1]) explain why the DC element vanishes from the secondary signal once saturation occurs. The attached COMTRADE record, captured during transformer energization with the HV side connected to a ring bus, illustrates this effect. Two sets of CTs were connected to terminals F3 and F7 of a GE T60 relay (only phase C from each CT, along with their summation, is shown in the snapshot). Under normal conditions, the DC component should decay gradually along an exponential curve (middle waveform – terminal F7). In contrast, the top waveform (terminal F3) initially follows the expected decay for a few cycles but then loses its DC component, leaving the signal more symmetrical around zero. At this point, a disturbance appears in the summation current (F3+F7), which causes the relay to trip. Therefore, when an event record shows a rapid loss of DC offset, CT saturation should be suspected. #FaultAnalysis #COMTRADE #CTSaturation #DecayingDC #GET60 #RingBus #Protection [1] Bogdan Kasztenny, Normann Fischer, D. Taylor, T. Prakash, and J. Jalli, “Do CTs  like DC? Performance of Current Transformers With Geomagnetically  Induced Currents,” proceedings of the 69th Annual Conference for  Protective Relay Engineers, College Station, TX, April 2016.

#Subsidence Current in Current Transformers (CTs): What is Subsidence Current? Subsidence current is a decaying DC component that appears in the secondary winding of a Current Transformer (CT) after the primary current is suddenly interrupted or reduced. This occurs due to the residual flux left in the CT’s magnetic core. #Why Does Subsidence Current Occur? 1. Saturation of the CT Core When a CT operates under high current conditions, its core may become magnetized. When the fault is cleared, the flux does not instantly reset to zero, causing a DC component in the secondary winding. 2. Remanence (Residual Magnetism) in the CT Core CT cores, especially those with high remanence, retain some magnetization after a current is removed. This residual magnetization causes a slow decay of the subsidence current over time. 3. Sudden Interruption of Primary Current When a circuit breaker trips during a fault, the primary current drops instantly to zero. However, the CT secondary winding cannot instantly stop conducting due to the stored magnetic energy, leading to subsidence current. #Effects of Subsidence Current ✅ False Operation of Protection Relays Protection relays, such as differential and overcurrent relays, may detect the decaying subsidence current as a fault and trip incorrectly. ✅ Delayed Resetting of Protection Relays Some relays require the current to fully decay before resetting. Subsidence current delays this process. ✅ Accuracy Issues in Measurement Subsidence current can distort CT secondary signals, leading to incorrect measurements in meters and protective relays. #How to Reduce Subsidence Current? 1. Using Low-Remanence CT Cores CT cores made of low-remanence materials (e.g., nanocrystalline or special alloys) reduce the retained magnetization. 2. Air Gaps in CT Cores Some CTs are designed with small air gaps in the core to reduce residual flux. 3. Demagnetization of CTs Applying AC excitation and gradually reducing it to zero can help remove residual flux in a CT core. 4. Using Numerical Relays with Filtering Algorithms Modern digital relays can filter out DC components (subsidence current) such as zero crossing detector to avoid false trips .