How can you prevent CT saturation in a power transmission protection system?

1 What causes CT saturation?

CT saturation is mainly caused by two factors: the magnitude and duration of the primary current, and the characteristics of the CT itself. The primary current can be high and long-lasting due to faults, inrush currents, or load switching. The CT characteristics include the rated burden, the knee-point voltage, the remanent flux, and the accuracy class. The rated burden is the maximum load that the CT can drive without exceeding a specified voltage drop. The knee-point voltage is the point where the secondary voltage becomes nonlinear with respect to the primary current. The remanent flux is the residual magnetic flux in the core after the primary current is interrupted. The accuracy class is the ratio of the error in the secondary current to the primary current under normal conditions.

2 How to select the right CT?

One of the most important steps to prevent CT saturation is to select the right CT for your protection system. You need to consider the following aspects: the fault current level, the relay type and setting, the CT ratio, the lead length and resistance, and the safety factor. The fault current level is the maximum current that the CT will experience during a fault. The relay type and setting determine the operating current and time of the protection device. The CT ratio is the ratio of the primary current to the secondary current. The lead length and resistance affect the voltage drop and the burden on the CT. The safety factor is the margin of error that allows the CT to operate within its accuracy class.

3 How to test the CT performance?

Another essential step to prevent CT saturation is to test the CT performance before and after installation. You need to perform the following tests: the polarity test, the ratio test, the burden test, and the saturation test. The polarity test checks whether the CT terminals are correctly marked and connected. The ratio test verifies whether the CT ratio matches the nameplate value. The burden test measures the actual burden on the CT secondary circuit. The saturation test evaluates the knee-point voltage and the excitation curve of the CT.

5 How to troubleshoot the CT saturation?

If you suspect that your protection system is affected by CT saturation, you need to troubleshoot the problem and identify the root cause. You can use the following methods: the event analysis, the fault recording, the oscillography, and the simulation. The event analysis reviews the sequence of events and the status of the protection devices during a fault. The fault recording captures the primary and secondary currents and voltages during a fault. The oscillography displays the waveform and the phase angle of the secondary current and voltage. The simulation reproduces the fault scenario and the CT behavior using software tools.

CT saturation is a common and challenging issue in power transmission protection systems. By following the steps and tips in this article, you can prevent CT saturation and ensure reliable and secure protection of your power transmission system.