This article demonstrates how CT saturation can adversely affect transformer differential protection and how to easily mitigate its effects via inbuilt relay functions. The solution is to provide calculations and simulation for a three-phase fault on the low side of the transformer just external to the CTs.
Application
The application is for a three-phase delta-wye connected transformer serving station load at a power plant.
Transformer Name Plate Data
Size = 1.5 MVA
XT = 6.77% on transformer base
kVwdg1 = 12.47 kV
kVwdg2 = 480 V
CTR1 = 500:5
CTR2 = 4,000:5
CT2 Data from Manufacturer Data Sheets
RCT = 1.861 ohms
(0.002326 ohms per turn)
VS = 190 V (saturation voltage at excitation current equal to 10 amps)
System Data
X/R = 6.777
Fault Current Calculations
Calculate the low side fault current as follows (ignoring system impedance for worst case scenario):
Figure 1: Low-Side Fault Current Calculations
Simulation
Obtain the corresponding secondary current using a CT saturation simulation.
Figure 2: CT Simulation Results for Full DC Offset
Observations
The secondary current is most distorted during the first cycle of the fault. To help mitigate unwanted tripping during the external fault condition, enable harmonic restraint for the transformer differential protection.
Figure 3: Harmonic Restraint
Harmonic blocking is not enabled since it can delay tripping during internal faults. Note: The relay vendor’s specifications for this particular relay indicate the transformer differential protection operates slower when harmonically restrained, which can also help ride through the period of saturation.
Figure 4: Transformer Differential Protection Operating Times
Conclusion
CT saturation can occur during faults external to transformer differential protection. Use harmonic restraint to help mitigate unwanted tripping during such events. Test the transformer protection relay to ensure the harmonic restraint works properly.
Steve Turner is in charge of system protection for the Fossil Generation Department at Arizona Public Service Company in Phoenix. After working with Beckwith Electric Company, Inc. for 10 years, Steve spent two years as a consultant in San Diego. His previous experience includes positions as an Application Engineer at GEC Alstom and in the international market for SEL focusing on transmission line protection applications. Steve also worked for Duke Energy (formerly Progress Energy), where he developed the first patent for double-ended fault location on overhead high-voltage transmission lines and was in charge of all maintenance standards in the transmission department for protective relaying. Steve has BSEE and MSEE degrees from Virginia Tech University. He has presented at numerous conferences including Georgia Tech Protective Relay Conference, Western Protective Relay Conference, ECNE, and Doble User Groups, as well as various international conferences. Steve is a senior member of IEEE and a member of the IEEE PSRC.