Current transformers (CTs) play a critical role in protection and metering. The proper operation of protection relays depends on CT accuracy to detect faults. Additionally, the accuracy of CT is more important for metering purposes. However, when a CT experiences core saturation, its secondary output no longer represents the true replica of the primary current. This distorts the secondary current waveform. Obviously, this distorted secondary current can lead to severe protection maloperation. Additionally, CT saturation can lead to misleading measurements of current and power.
What is CT Saturation?
Obviously, a current transformer works on the principle of electromagnetic induction. Its core must remain in the linear region of the magnetization curve for proper performance.
CT saturation occurs when the magnetic core approaches its knee point. After the saturation, a large increase in magnetizing force (current) results in a smaller increase in flux density. As a result, the core cannot efficiently carry additional magnetic flux. Since the flux becomes stagnant, it can not increase linearly with the primary current. Therefore, the rate of change of flux linkage with respect to time reduces abruptly. So, the CT cannot induce the required secondary voltage. Rather, the CT secondary voltage reduces even below the normal level. Since this secondary voltage is responsible for the secondary current, the secondary current becomes reduced, clipped, and also distorted.
CT saturation results from several electrical and system-related factors. Understanding these causes is crucial for designing an accurate protection system.
Causes of CT Core Saturation
High Fault Current (Primary Overcurrent)
During a short circuit, the primary current can surge to 20–40 times the rated current. This high primary current produces high flux in the core. Sometimes the flux exceeds the flux-carrying capacity of the CT core. As a result, the CT core enters saturation. From the other point of view, the secondary induced voltage can not exceed the knee voltage of the CT. However, to reproduce this short-circuit primary current, the CT must generate a high secondary voltage.
High Burden on Secondary Circuit
The total burden includes relay input impedance, lead resistance (copper length), terminal connectors, and other additional devices such as meters and test switches. These impedances and CT secondary impedances form a closed loop. If we apply the KVL (Kirchhoff Voltage Law) the voltage drop across the connected burden equals and opposes the induced voltage across the CT secondary winding. So, if the connected burden is high, very easily the burden voltage crosses the knee voltage. Obviously, this pushes CT toward saturation.
DC Offset in Fault Current
When a fault occurs at a non–zero crossing of the AC waveform, the current contains DC offset. This offset adds extra flux to the core. This flux also tries to offset in the core. Hence, this may cause asymmetric flux distribution in the core. Therefore, this pushes the CT deeper into saturation, especially for the first few cycles. This is one of the most common reasons for CT saturation during fault.
Residual Flux in the Core
Current transformer cores can retain magnetic flux after previous fault conditions. So, this residual flux reduces the room for flux to be created during the next fault. Obviously, this may also saturate the core well before.
Low Accuracy Class or Undersized CT
The metering core of a CT can become saturated just above 110 to 120% of the rated current. Because, a metering CT is supposed to measure the current up to maximum 120% range of rated current. So, if we use a metering CT core for protection mistakenly the CT core goes saturation much earlier. In contrast, a protection CT core can withstand much higher secondary voltages without saturation. In other words, a protection CT can carry much higher secondary currents without saturation.
Incorrect Polarity Connections or Open Circuit
An open-circuited CT develops extremely high voltages, risking damage and forcing unstable core behavior. Although not exactly the same as saturation, it leads to invalid secondary outputs.