transformer overexcitation protection (ansi 24)

Over-excitation protection (also known as Over fluxing protection) is designed to protect a transformer's magnetic core and structural components from thermal damage caused by excessive magnetic flux density.

1. The Physics: Why does it happen?

The magnetic flux (Phi) in a transformer core is determined by the ratio of the applied voltage (V) to the frequency (f). This relationship is derived from Faraday’s Law: Rearranging for flux density (B), which is proportional to flux :

The Saturation Problem

Transformers are designed to operate at a specific flux density (typically near the "knee" of the saturation curve).

• If Voltage increases or Frequency decreases, the flux density rises.

• When the core saturates, it can no longer contain the magnetic flux within the laminations.

• This "stray flux" escapes into non-laminated metal parts (like the tank, bolts, and frames), causing massive eddy current losses.

• Result: Rapid, localized heating that can destroy insulation and the core itself within seconds or minutes.

2. Common Causes

Overexcitation typically occurs during abnormal system conditions rather than internal faults:

• • Generator Startup/Shutdown: If the excitation system (AVR) is active while the turbine is at low speed (low frequency).

  • Load Shedding: A sudden loss of a large load can cause the system voltage to spike.

  • Solar Storms: Geomagnetically Induced Currents (GIC) can cause "half-cycle saturation."

  • Tap Changer Malfunction: Incorrectly setting a tap can lead to overvoltage on specific windings.

3. The Protection Logic (ANSI 24)

Relays monitor the V/Hz ratio. Since different levels of overexcitation can be tolerated for different lengths of time, the protection usually follows an Inverse Time Characteristic.

Typical Settings

1. Alarm/Warning Stage : Often set at 1.05 to 1.10 p.u. (per unit) of the rated V/Hz.

2. Trip Stage (Inverse Time) : Follows a curve provided by the manufacturer. A typical transformer can withstand 1.10 p.u. continuously but might only last 60 seconds at 1.25 p.u.

3. High-Level Trip (Definite Time) : A "fast trip  for extreme conditions (e.g., >1.40 p.u.) to prevent immediate catastrophic failure.

4. Key Implementation Rules

• Winding Selection : The protection should be connected to the side of the transformer that experiences the most voltage variation, usually the side without the Load Tap Changer (LTC), as the LTC can mask the true flux levels in the core.

• Differential Blocking : During overexcitation, the magnetizing current increases significantly. This can look like an internal fault to a Differential Relay (ANSI 87T). Modern relays use 5th harmonic unrestraint/blocking to prevent the differential protection from tripping incorrectly during an over fluxing event.