Effect of CT Saturation on Transformer Differential Protection

Steve Turner, Arizona Public Service CompanyColumns, Relay Column, Winter 2023 Columns

This article demonstrates how current transformer (CT) saturation can adversely affect transformer differential protection and how to easily mitigate its effects via inbuilt relay functions. We provide calculations and simulation for a three-phase fault on the low side of the transformer just external to the CTs.


The application is for a three-phase delta-wye connected transformer serving station 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 = 4000:5

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 shown in Figure 1 (ignoring system impedance for worst-case scenario).

Figure 1: Low-Side Fault Current Calculations


Obtain the corresponding secondary current using a CT saturation simulation (Figure 2).

Figure 2: CT Simulation Results for Full DC Offset


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 (Figure 3) for the transformer differential protection.

Figure 3: Harmonic Restraint

Harmonic blocking is not enabled since it can delay tripping during internal faults. A note from the relay vendor specifications for this particular relay states that the transformer differential protection operates slower when harmonically restrained, which can also help ride through the period of saturation.


CT saturation can occur during faults external to transformer differential protection. Using harmonic restraint can 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. Steve worked as a consultant for two years, and held positions at Beckwith Electric Company, GEC Alstom, SEL, and Duke Energy, where he developed the first patent for double-ended fault location on overhead high-voltage transmission lines and was in charge of maintenance standards in the transmission department for protective relaying. Steve has BSEE and MSEE degrees from Virginia Tech University. Steve is an IEEE Senior Member and a member of the IEEE PSRC, and has presented at numerous conferences.