The transformer plays a vital role in reliable transmission and distribution of power. It is a critical link in industrial and domestic power system network. It is a simple, static and robust machine utilized for the multidimensional development of infrastructure from urban to rural. Although it is a simple induction machine, some critical issues and typical phenomena are associated with it.
Typical Phenomena in Transformers
Some typical phenomena in transformers are elaborated below:
Over-excitation or over fluxing and generation of harmonics
Magnetization Inrush and Harmonic Current
Over-excitation or over fluxing and generation of harmonics
Normally, a transformer gets over-excitation due to over voltage or under frequency. In this scenario, excitation current increases sharply and the transformer will tend to become overheated due to increased excitation current, hysteresis losses, and eddy currents. The transformer may get damaged if this condition is sustained.
As shown in figure-1, for an overvoltage of 20%, the excitation current can increase about 10 times of normal excitation current. For a higher overvoltage, the excitation current can increase above the pickup level of a differential relay unrestrained for excitation current.
Figure 1: Transformer Excitation Current Vs Excitation Voltage
The increased excitation current produces operating current in the differential relay, but an operation of this relay is not desirable since the immediate response is not necessary. The power system should be allowed time to correct itself.
The operation of a differential relay indicates a transformer failure. Therefore, after operation of a differential relay investigation of a transformer always shall be done. If the relay has operated during an over-excitation condition, additional valuable time for investigation of the transformer would then be lost.
An analysis of the current during an overexcitation condition shows a predominant 5th harmonic component. A typical example for a modern transformer is provided in fig. 1. This can be utilized to identify an over magnetizing condition. The differential relay is, therefore, provided with a 5th harmonic restraint to prevent the relay from operation during an overexcitation condition of a transformer due to over voltage. Transformer likely to be exposed to overvoltage or under- frequency conditions should be provided with a V/Hz relay,
Magnetization Inrush and Harmonic Current
A Phenomenon-Magnetization current
A transformer inrush event is actually magnetizing inrush current. Since in the transformers windings are magnetically coupled by the flux, therefore, on increasing the excitation voltage, flux increases. In this situation to maintain this additional flux, transformer draws more current from the source. This additional current would be inrush current, required for a magnetizing branch of the transformer as shown in the figure 2.
Inrush current is more predominant in one winding, results in differential current lead to operation of differential protection.
During transformer charging event, an inrush current flows only in one winding, but magnetizing current may appear in both winding. In the above figure, the current I1 is the inrush current.
Characteristics of Magnetic Inrush Current
The inrush current has the following features:
It can appear in all three phases and a grounded neutral.
Its magnitude is always different in all three phases as well as in the neutral.
Transformer with oriented core steel lamination, if energized from high voltage side, the magnitude of inrush current would be 5-10 times of rated current while from low voltage side it could be 10 to 20 time of its rated current.
The shape of the inrush current for a delta connected transformer will not be the same as for a Y-connected transformer.
The inrush current has a significant DC component and is also rich in harmonics.
Second harmonic is predominant harmonic in the inrush current.
Events Generate Inrush current
The inrush current can be produced during the following events:
Transformer Energization
In concern of magnetizing inrush currents, transformer charging is a crucial event.
Magnetizing inrush current during fault clearing
During any external fault system voltages reduces significantly. Therefore, transformer excitation voltage reduces. This excitation voltage will be recovered when this fault is cleared. Recovery of the voltage will force a DC offset on the flux linkages, resulting in magnetizing inrush current. In this case, residual flux is not available in the core. Therefore, magnetizing inrush current will be less than that of energization. The current measured by the differential relay will be fairly linear due to the presence of load current and may result in low level of second harmonic current.
Sympathetic inrush
As shown in the figure 3, when an un-energized transformer TR-2, connected parallel to an energized transformer TR-1, is charged from the source, sympathetic magnetizing inrush currents flows in an energized transformer TR-1.
On energizing the second transformer TR-2, a voltage drop appears across the impedance of power supply line of the transformers. As a consequence of voltage drop, the core of transformer TR-1 will be saturated in the negative direction. This saturation causes magnetizing inrush current will flow to supply the flux.
The inrush current in the parallel transformer will have a phase shift of 180 degrees. The magnitude of the magnetizing inrush current is generally not as severe as the other cases.
Factors Affecting Inrush Current
The shape, magnitude, and duration of the inrush current depend on the following characteristic factors:
Source impedance
Size of the power transformer
Moments of transformer switch-in
Residual flux in the core
Magnetically efficient transformer core material
Content of inrush current
Source Impedance
The location and physical installation of the transformer also play an important role to influence the magnetizing inrush current. The excitation voltage at the transformer terminals is the system source voltage minus the voltage drop across the system impedance. Therefore, as the source impedance decreases, transformer excitation voltage increases which is a characteristic of stronger sources. As a consequence of this, the magnitude of the inrush current increases.
Size of Power Transformer
The impedance of the power transformer is also an important factor to control the decay of the inrush current over time. The time constant of the circuit (L/R) is not constant because L is variable due to change in permeability of the core material. If system resistance is high, the value of the time constant (L/R) will be low. Thus, inrush current will decay more rapidly. The time constant of the inrush current is 0.2 to 1 minutes depending upon whether the transformer is small or large.
Moments of Transformer Switch-in
At the moment of transformer switching, the magnitude of the inrush current depends on the scenario of applied voltage, the available residual flux in the core and phase angle difference between them.
The magnetizing inrush current will be maximum, if the moment of switching occurs at the zero crossing of the voltage, with zero phase difference between residual flux and flux due to inrush current. Thus, both fluxes will be added due to the same direction. As a consequence, the core would be saturated, and inrush current would be increased under the constraint of the source and residual impedance of the transformer.
If both fluxes get the opposite direction, there will be no saturation of the core, and the magnetizing inrush current will be minimum or tends to zero.
Residual Flux in Core
When excitation voltage of the transformer is removed, some level of flux remains in the transformer core, this flux level is called of Remnant flux or Residual flux. It can be found from the magnetic hysteresis loop of the transformer core. Its value can be 30% to 80% of the maximum flux in the core with a positive or negative sign. When the transformer is energized, residual flux is added to the flux generated by the excitation voltage. Therefore, the flux equation becomes,
Transformer core saturation depends on the sign of the residual flux. Once the core is fully saturated, the residual flux will not be effective.
Magnetically Efficient
Transformer Core
To reduce the losses in the core, some significant changes are being incorporated in transformer core designing.
Use of high permeability electrical steel (High-B)
High permeability steel is magnetically very efficient, results in lower excitation currents and therefore lower inrush current.
Reduction of reluctance in the core
The air gap increases the reluctance of the core, thereby, reducing the magnetic efficiency of the core. Laminations are now constructed such that they overlap each other to provide a continuous path for the flux. This construction reduces the reluctance in the core, and therefore, increases the flux density and reduces the excitation current.
Use of larger cross-sectional area core
To limit losses, transformers are designed with lower flux densities. The flux density is limited by using a core with a larger crosssectional area. As a consequence of larger crosssectional area, level of excitation current as well as magnetizing inrush current reduces.
Content of Inrush Current
The initial inrush of magnetic current has a high component of even and odd harmonic, as shown in the table 1 obtained by an event recorder.
It can be observed from the above recorded event, in inrush current, 2nd order harmonic is the predominant component. This property of inrush magnetizing current is used for restraining the relay operation during initial inrush of magnetizing current.
Effects of Magnetization Inrush current
In the operation of an electric power system, charging of large power transformers is considered as a critical event. When a transformer is charged by the grid or utility, it draws very high magnitude of the current, known as inrush current, the typical value of this current could be ten to twelve times.
This high magnitude inrush current produces many problems like mechanical stress on transformer & harmonics injection towards generator or grid, malfunctioning of system protection, etc. The main reason behind this criticality is a generation of the unpredictable system transients during charging of transformers.
Effect of Inrush Current on Differential Protection
Differential protection is the standard protection used to protect transformers. It compares entering and leaving currents the transformer to create a differential current.
In an ideal case, when normal current is flowing through the transformer the differential current is zero. In an internal fault condition, a differential current is always greater than zero or restraining current therefore differential relay operates. Apart from ideal situation, there are two common situations when differential protection incorrectly gets differential current and operates the protection relay.
a) For faults outside the protection zone, if fault current is very high, CT gets saturated. Thus, the error in the measured signal of the saturated CT results in a significant error in the differential current leads to undesired operation of the differential element.
b) The other scenario depends on the switching operation and corresponding inrush event of the transformer. This inrush of current occurs only in one winding of the transformer. Therefore, it may produce a differential current that results in the operation of the differential protection.
Harmonic Blocking and Harmonic Restraint in Transformer Differential Protection
The main purpose of transformer inrush restraint function is to block the differential element from operation during an inrush event. It will permit the differential element to operate only during the event of an internal fault.
Since both events supply large differential current to the differential element, differentiation between inrush current and fault current is the great challenge. Here, traditional and latest generation methods adopted for their differentiation are briefed below:
Traditional Method – Harmonic Blocking & Restraint
This method works on the following assumptions:
• The magnetizing inrush current contains high levels of second harmonic current.
• Transformer’s internal fault current typically has very low levels of second harmonic current.
• The method compares the magnitude of the second harmonic current (100Hz) to fundamental frequency current (50Hz) in the differential current.
• When the ratio of the second harmonic component to the fundaments component is more than a second harmonic set point, thus the system would understand it as inrush phenomenon, and it will block to the differential operation.
• When the ratio of the second harmonic component to the fundaments component is less than a second harmonic set point, thus, the system would understand it as transformer internal fault phenomenon, and it will allow operating the differential operation.
Latest Generation Method – Harmonic Blocking & Restraint
In latest generation, harmonic restraint is a modified version of traditional harmonic restraint that considers the magnitude and phase of the second harmonic and fundamental frequency component in the differential current. Some inrush events initially produce low levels of the second harmonic in the differential current.
However, this method successfully restrains tripping when faced with low levels of second harmonic current during an inrush event. If some second harmonic is present in the internal fault current, this method may give slow tripping by a few cycles.
