Engineering Tutorial

Transformers Questions & Answers Part-1

Inrush Currents in Transformers – Causes

When a transformer is energized after a short interruption, the transformer may draw high inrush currents from the system due to core magnetization being out of synch with the voltage. The inrush currents will be as high as short circuit currents in the transformer (almost 20 to 40 times the rated normal full load current of transformer). Inrush currents may cause fuse, relays or re-closers to falsely operate. It may also falsely operate the faulted circuit indicators or cause sectionalizers to mis-operate.

When the transformer is switched in, if the system voltage and transformer core magnetization are not in synch, a magnetic transient may occurs. This transient may drive the core into saturation and drives a large amount current into the transformer causing transformer core to damage

Factors Significantly Impact Inrush Currents in Transformer:

A transformer that is designed to operate lower on the saturation curve draws less inrush currents as there is more margin between the saturation point and the normal operating. The extra flux during switching is less likely to push the core into saturation
Large transformers draw more inrush current. Large transformers will have smaller saturation impedance
Higher source impedance relative to the transformer size limits the currents that the transformer can pull from the system
The point where the circuit breaker close (position of flux wave in sine wave). The worst case will be when the flux is at maximum (peak) and voltage is minimum (in transformer the applied voltage lag behinds the flux by 90 deg).
Other factors have little significance. The load on the transformer does not significantly change the inrush currents. While switching transients cause high inrush, other voltage transients especially voltage transients with dc components can saturate the core of the transformer and cause inrush currents
A lightning flash near the transformer can drive the transformer core to saturation
When the nearby fault was cleared and transformer voltage is recovering from the voltage sag, the sudden rise in voltage can drive the transformer to saturation
Energizing a transformer can cause the nearby transformer to also draw inrush currents. The inrush currents into the switched transformer has a significant dc component that can cause the voltage drop. The dc component can push the other transformer into saturation and draws inrush.

Three Winding Transformers Advantages

Generally in power system mostly two winding transformers are employed. But three winding transformers are employed because of some advantages:

The most common reason for having a three winding transformer is to provide a delta connection tertiary winding
To limit the fault level on the low voltage system of the transformer by dividing the LV infeed (in order to provide double secondary windings)
Providing tertiary winding helps to interconnect different power system operating at different voltages (Three winding transformer helps provide power supply at two different secondary voltages, 220kV/11kV/6.6kV transformer can able to provide power at two different voltage levels (11kV and 6.6kV)
To regulate the voltage and reactive power of the system by providing synchronous capacitor connected to one of the terminals of the transformer.

Why Delta winding prefer:

It is always desirable to have one delta connection winding in the three phase transformer as delta connected three phase winding will offer low impedance path for the three phase currents. Also the presence of delta connected three phase winding allows to circulate the current around the delta winding in the event of unbalance loading condition.

Although power system designers aims to avoid use of star/star transformer in power system but cases will arise when the phase shift between the star/delta and delta/star is not applicable such as in the power station supplying power to auxiliary system. Therefore it is common practice to have a star/star with delta tertiary three winding transformer supplying power to the plant auxiliary system.

B/H Curve of the magnetic material (core of the transformer) is not linear. Is a sinusoidal voltage (flux) is applied across the primary winding, the magnetizing current obtained will not be sinusoidal in nature and consists of fundamental component and several harmonics. Third harmonic components predominate with several other higher harmonic components. If there is no delta connected winding, or if the star connections of the transformers are not grounded, the line to earth capacitance currents supply system lines supply the harmonic components. If the harmonic components cannot flow in any one of these paths then, secondary voltage will be distorted.

Magnetostriction: Noise In Transformer:

The basic cause for the noise in the transformer is due to magnetostriction of the sheets in the magnetic circuit (core of the transformer). Variations in the magnetic induction subjected to the sheets to periodic variations in the length, the amplitude which is in the order of microns per meter length.
The fundamental frequency with which these vibrations occur is double that of the system frequency, (for 60Hz frequency vibration frequency will be of the order of 120Hz) and also constitute numerous harmonics. Also various parts of the transformer, starting with the magnetic circuit (core) are liable to vibrate due to magnetostricition effect.
The noise generated due to magnetostricition effect transmitted from the magnetic circuit to the tank of the transformer either through direct conduction to supporting points or through the oil and insulating material used in transformer. The transformer tank and the radiator radiate the acoustic noise or energy in to the ambient atmosphere
Another source is due to the vibration of magnetic sheets perpendicular to the surface either at the edge or at the core packets, or at the joints between the leg and the yoke
The current carrying windings is also a source of noise, however the amplitude of the noise is very less and is not detectable.
Cooling fans and pumps employed for cooling the transformer is also acts as source of noise.

Methods to reduce Transformer noise:

The main source of noise in the transformer is due to magnetostriction effect of the magnetic circuit or core. In order to reduce the noise cold rolled grain-oriented plant, with low magnetostriction and improved flatness is employed
Ensuring uniform flux distribution and reduction in the cross flux also reduces the noise
Elimination of the clamp bolt holes, use of resin impregnated glass-fibre bands instead of core bolts, gluing of core packets can reduce the noise

These above specified remedial methods not only reduces the noise level by 5 to 10 dB, but also reduces the losses and no-load current.

Different Transformer Internal Faults

Faults in Transformers:

Some of the faults in the transformers are likely to be over-voltages which resulting from the atmospheric phenomenon (lightning) transmitted by overhead lines.
Switching in the power system (especially high voltage switching more than 400kV) can produce over-voltages of less steep but longer duration surges stressing both liquid and solid dielectrics (insulation). These over-voltages should be restricted in amplitude to a value below the transformer insulation breakdown withstanding level.
Short circuits in the power system subject the transformer to currents of 10 to 20 times the rated currents (short circuit current level will be severe when fault occur close to transformer). Power transformer is generally designed to withstand tens of short circuits, lasting not more than 2 sec duration in its life time. If there are more short circuits than the designed limit special construction is required. Short circuits should eliminated (by isolating faulty power system by opening circuit breakers) as quickly as possible to limit the short circuit intensity on transformer.
Overloads can arise in transformers from planned or fortuitous (unexpected) circumstances. In the first case, temperature increase in transformer insulating material should not exceed the standard value. In the second case, certain time limit can be tolerated but this will have certain cost in reduction in the life of the transformer.

Internal Faults in Transformer:

Electrodynamic faults: which occurs between insulation and current carrying conductors, HV and LV winding due to external and internal short circuits
Electromagnetic faults: Which occur due to eddy currents induced in the magnetic circuits or the clamping structure.
Electrical faults which occurs due to bad contacts in the leads or bad contacts in the tap changer Dielectric faults: Which occur due to shorting between windings or between live parts and earth, partial discharges
Thermal faults: Which occurs due to abnormal temperature rise, hot spot, thermal ageing or pollution in transformer oil
Mechanical faults: Which occur due to vibrations, leakages or defective operation of the tap changers

Different types of defects originated in transformers will have degree of gravity depending on the amount of damage it can do on transformers and their consequences. Some of the defects (vibrations, partial discharges) will not immediately endanger the equipment but care must be taken before causing major damage. On the other hand, defects such as (over-voltages and short circuits, and initial breakdown) requires immediate attention.

Transformer Parallel operation conditions

Parallel operation of transformers is required in cases such as the power to be delivered is more than the individual transformer rating. In such cases operating two or more transformers to facilitate the power flow is possible, but certain conditions to be followed while operating transformers in parallel condition

Conditions for parallel operation of Transformers:

Polarities of the transformers must be same (wrong polarity leads to dead short)
The voltage rating of both primary and secondaries are identical. This means the that the transformation or turns ratio must be same for the transformers which are operated to be parallel. (Voltage ratio is to be maintained to avoid circulation current)
Percent (or per unit ) impedance of the transformers are to be same in magnitude and should have the same phase angle (X/R ratio should be same for transformers operating in parallel else division of load will not be proportional to the kVA ratings of transformers)
Phase displacement between the primary and secondary line voltages of the transformers should be same (transformers of star/star and delta/star cannot be paralleled because of the phase difference of -30^o making paralleling impossible (cannot be compensated))
Phase sequence of the transformers should be same. (Phase sequence is the order in which the terminal voltage attains their maximum value. Therefore in paralleling the two three phase transformers those terminals whose voltage attains maximum values simultaneously must be paired up.

Conditions 1,4 and 5 are absolutely essential and must be fulfilled, condition 2 must be satisfied to a close degree and condition 3 (X/R ratio) must be satisfied in order to have equal loading on the transformers.

When DC supply given to Transformer what happens ?

DC supply to Transformer:

A Transformer cannot be operated on the DC source or never connected to DC supply. If a rated dc voltage is applied to the primary of the transformer, the flux produced in the transformer core will not vary but remain constant in magnitude. So therefore no emf is induced in the secondary winding except during the moment of switching on the dc supply. As no induced emf is produced current cannot be delivered from the secondary side to the load.

Also the flux flowing through the iron core from primary winding to secondary winding not only links the secondary winding but also primary winding. Due to this flux linkage self induced emf is produced in the primary winding. This self induced emf in the primary winding will oppose the applied voltage and hence it acts as back emf. This back emf limits the primary current flowing through the primary winding in normal operating condition (similar like dc machine armature current).

When a dc supply is provided to the transformer primary no self induced emf will be generated (no back emf). Therefore heavy current will flow in the transformer primary winding which may result in burning down the transformer primary winding.