IEEE Std C37.91-2021 

Protecting Power Transformers

6.4.1 Internal Faults

 

6. Relay Currents

6.4 Performance of CTs

6.4.1 Internal Faults 

During a fault condition in a transformer, such as an internal fault or a fault within the protected zone of the relay system, the current transformers (CTs) used in the relay system may experience saturation. CT saturation occurs when the magnetic core of the CT is unable to handle the high levels of current flowing through it, resulting in distorted current waveforms.

Severe CT saturation can have an impact on the operation of the transformer differential relay. The relay may fail to operate or experience delays in its operation due to the effects of CT saturation. This depends on how the relay responds to distorted currents.

There are two types of CT saturation: asymmetrical saturation and symmetrical saturation.

 

This occurs when the fault current has a direct current (dc) offset. The dc offset can cause the CTs to saturate temporarily until the dc transient decays and the current waveform becomes symmetrical again.

During asymmetrical saturation, second and third harmonics are predominant initially. Even-numbered harmonics, which are also present in inrush currents, can cause issues with even-numbered harmonic restraint or blocking techniques.

As the offset dc component of the short-circuit current decays, the even-numbered harmonics disappear. The presence of odd-numbered harmonics depends on whether the CT experiences symmetrical saturation.

2. Symmetrical Saturation: 

This type of saturation occurs when a CT is severely underrated, meaning it is not capable of handling the fault current levels regardless of the presence of a dc transient. In symmetrical saturation, both even-numbered and odd-numbered harmonics can be present.

To mitigate the effects of saturation, the CTs used in the relay system are typically rated to avoid symmetrical saturation for internal faults and limit asymmetrical saturation as much as possible for through-faults. However, for severe internal faults where saturation cannot be completely avoided, a high-set unrestrained element (87U) is often employed to provide dependability in relay operation.

It is important to carefully select and size the CTs based on the expected fault current levels to ensure reliable operation of the transformer differential relay and minimize the impact of saturation.

What is DC offset?

DC offset refers to a situation where there is an imbalance between the positive and negative half-cycles of an alternating current (AC) waveform, causing the waveform to have a non-zero average value. This can occur in power systems due to various reasons such as switching operations, faults, or the presence of rectifiers and other non-linear loads.

 

In power systems, the presence of DC offset can have several adverse effects:

The DC offset causes distortion in the AC waveforms, which can lead to harmonics and affect the quality of power.

Measurement equipment like current transformers and energy meters can give inaccurate readings when there is a DC offset because these devices are typically calibrated for pure AC waveforms.

The presence of a DC component in the AC waveform can cause increased heating in electrical equipment like transformers and induction motors. This is because the DC component causes a continuous flow of current that can result in additional losses and thermal stress.

Protection relays that are designed to protect the power system against faults may malfunction due to DC offset. Some relays are sensitive to the DC component and may trip erroneously or fail to trip when required.

In transformers, a DC offset in the current waveform can cause magnetizing inrush currents, which are transient currents that can be significantly larger than the normal operating current. This can cause mechanical stress and heating in the transformer.

Chris Werstiuk has a very good article here. https://relaytraining.com/what-is-dc-offset-ask-chris/