Protection webinars with Prof. Dr. Lemmer - Current transformer saturation

 

Protection Current Transformer Saturation

Protection current transformers (CTs) play a critical role in power system protection by accurately stepping down high currents for relays and meters. However, under fault conditions, CTs can experience saturation, leading to inaccurate current measurements and potential maloperation of protection relays.

Causes of CT Saturation

1. High Fault Currents – When fault currents exceed the CT’s designed capability, the magnetic core reaches its limit, distorting the secondary current.
2. Residual Flux – If a CT is not properly demagnetized after a fault, leftover flux can cause early saturation.
3. Burden Overload – Excessive burden (relay, wiring resistance) increases voltage demand, pushing the CT into saturation.
4. DC Offset in Fault Current – Asymmetrical faults introduce a DC component, shifting the CT core deeper into saturation.

Effects of CT Saturation

• Incorrect Relay Operation – Protective relays may under-trip or over-trip due to distorted current signals.
• Delayed Fault Detection – Some relay algorithms struggle to detect faults if the CT output is distorted.
• Harmonic Distortion – Saturation introduces harmonics that can interfere with digital protection schemes.

Mitigation Strategies

• Using High-Accuracy Class CTs – CTs with high knee-point voltage and lower saturation tendency are preferred.
• Proper CT Sizing – Selecting CTs with sufficient VA capacity and a high saturation knee-point voltage minimizes risks.
• Air-Core CTs or Rogowski Coils – These alternatives do not saturate and can improve fault detection accuracy.
• Numerical Relays with CT Saturation Detection – Modern relays incorporate algorithms to recognize and compensate for saturation effects.

Preventing CT saturation is crucial for maintaining reliable power system protection, reducing the risk of relay misoperation, and ensuring quick fault clearance.

Protection Current Transformer Saturation and Example

What is Current Transformer (CT) Saturation?

In power systems, protection current transformers (CTs) step down high currents to measurable levels for relays and meters. However, when the primary current exceeds a certain level, the CT core may saturate, meaning it can no longer accurately transform the current. This leads to distorted secondary currents, affecting relay performance and possibly causing maloperation or failure of protective devices.

CT Saturation Point

The saturation point is the primary current level beyond which the CT cannot maintain accurate transformation. It depends on:

• Core material and design
• Burden (connected load) on the CT
• Magnetizing characteristics
• Primary current magnitude and duration

A CT reaches saturation when the core’s magnetic flux exceeds its designed limit, causing waveform distortion and affecting protection accuracy.

Example of CT Saturation in Protection

Consider a 2000/5A protection CT with a knee-point voltage of 400V and a connected burden of 5 ohms. If a fault causes 20 times the rated current (40,000A primary, 100A secondary), the secondary voltage required is:

$$V = I \times R = 100A \times 5\Omega = 500V$$

Since 500V exceeds the knee-point voltage (400V), the CT saturates, leading to:

• Distorted secondary current
• Relay misoperation (delayed or failure to trip)
• Protection failure

Mitigation of CT Saturation

• Use high knee-point voltage CTs for protection applications
• Reduce burden to lower secondary voltage requirements
• Employ numerical relays with saturation detection algorithms
• Use flux balancing techniques or air-gapped CT cores

Understanding and mitigating CT saturation ensures reliable protection system performance, preventing faults from escalating into major power system failures.

Current Transformer Saturation: Explanation with Example

What is Current Transformer (CT) Saturation?
Current transformer saturation occurs when the magnetic core of the CT is unable to accurately transform high currents due to excessive magnetic flux. This results in distorted secondary current, leading to inaccurate readings and potential protection failures.

Example of CT Saturation:
Consider a CT with a 500:5 ratio used in a power system. During normal operation, a primary current of 500A corresponds to 5A in the secondary winding. However, during a short circuit event, the primary current might surge to 5000A. If the CT is not designed for such high fault currents, its core may saturate.

When saturation happens:

• The secondary current no longer follows the primary current proportionally.
• The output waveform distorts, reducing the effectiveness of protective relays.
• The protection system might delay or fail to trip, causing equipment damage.

Preventing CT Saturation:

• Choosing Proper CT Ratings: Ensure the CT's knee-point voltage and burden are suitable for high fault conditions.
• Using High-Accuracy Class CTs: Opt for CTs with a lower saturation tendency (e.g., Class C or TPX, TPY, TPZ).
• Implementing Auxiliary Protection: Differential and flux-based protection can compensate for CT saturation effects.

Understanding and mitigating CT saturation is critical for reliable power system protection and operation.