• high amplitude of inrush current at start-up of power transformer,
• incorrect operation of protective relays,
• the windings are exposed to mechanical stresses that can damage the transformer
• power-quality problems may arise: high resonant harmonic overvoltages and voltage sags
• erroneous diagnostic electrical measurements on a transformer:
•    -  Frequency Response Analysis Test graphs with resonant peaks at the wrong place [6],
•    -  Excitation Current Test showing incorrect pattern of current readings (pattern is used for diagnostics)

Kovan et al. in their paper [2] conclude that when the mitigation technique was used, the inrush currents calculated at the head of the feeder were reduced by about 60%.  The IEEE PC57.102 recommends removing remanency before putting a power transformer back in service.

DEMAGNETIZATION

METHODS

Several methods of removing remanent magnetism are described in the literature. The oldest ones back in 1960ties and 1970ties were performed using car battery, creating a large arc as the cables were disconnected from the transformer bushings. This method would demagnetize one phase and magnetize the other phase of a three phase unit. Also, the arc would be a scary event, that could cause an accident if not from the arc itself , then by the fall from a transformer top. This method, most commonly used for demagnetization of a transformer is based on the standard found in IEEE 62-1995 (section 6.1.3.5) (6) which directs one to alternate the polarity of a fixed voltage with decreasing application time per alternation of polarity. With each alternation, the voltage is applied until the current flow has reversed and is “slightly lower” in absolute magnitude than the current in the previous application. The time was used as a measure of the current magnitude, given the car battery voltage is constant.

Makowski in his thesis [3] lists three methods: Permeability Method, Time Based Method, and Integration Method. We will list other methods as well:

A. Permeability Method was expected to be the most direct and quickest demagnetization method since it only required the voltage to be applied once for saturation and then reversed once for demagnetization. However, its accuracy and effectiveness regarding demagnetization is dependent on many assumptions about the properties
of the transformer.

B. Time based,  where the magnetic flux is directly proportional to the amount of time that a constant voltage is applied to the winding, by measuring the time needed
for the magnetic state of the core to switch from being saturated in one direction to becoming saturated in the opposite direction we can determine how long a constant
voltage needs to be applied to the winding that is saturated in order to reach the neutral point.

C. Integration method calculates the integral of the voltage applied in time, and provides as a result the exact time required to demagnetize the core applying reversed polarity.

D. The Controlled Current method is the "engineering" approach to the IEEE standard directive to alternate polarity and lower the flux in the magnetic core by controlling the current amplitude in each step. The value of the initial amplitude is not important as long the saturation is reached on all three legs of the magnetic core. Following initial current charging and discharging, the next alteration is of 60% magnitude and when reached, discharge is initiated and the steps go on in succession until the very small value of Ampere-turns is applied as the very last step.

E. In the paper by Kovan [2] a different procedure from the Integration one explained under C above is given. The steps to demagnetize a core are as follows:
1) Bring the core to saturation by applying a positive voltage. This step is needed because the initial residual flux is not known. The indication that the core has
reached (positive) saturation is when the current stops increasing.
2) Reverse the applied voltage (to negative ) and measure the time that it takes for the core to be fully saturated in the reverse direction. The time to bring the core from positive saturation to negative saturation is "T". According to Faraday’s Law, the integral of the voltage from [+] to [-] gives a flux equal to twice the saturation flux.
3) Reverse the voltage once again to apply positive Vdc . Theoretically, if we apply the voltage for a time equal to "T"/2 starting from negative saturation , the core would be completely demagnetized.

Demagnetization of 1100MVA transformer following the Controlled current method

EVALUATING DEMAGNETIZATION SUCCESS

To verify that the demagnetization was performed successfully, one has two options: to perform the FRA test and compare the graph with the one obtained in the demagnetized state, or to perform the single phase Excitation current measurement (Iex) and compare the values of test current phase per phase to the known value. In case no known values are available, looking at the Iex pattern and knowing the construction of the magnetic core may be helpful.

The single phase excitation current values for a demagnetized three phase transformer with three legged core form magnetic circuit should compare between outer phases (A and C). When magnetized, these currents measured at 10kV test voltage, can show over 25% difference [4].