1). What is a Transformer?
A transformer is a static electrical device that transfers electrical energy from one circuit to another through electromagnetic induction. It operates without changing the frequency of the power supply. Transformers are primarily used to step up (increase) or step down (decrease) voltage levels.
2). What is the Principle Behind a Transformer’s Operation?
A transformer operates on the principle of electromagnetic induction, also known as Faraday’s Law of Electromagnetic Induction. It transfers electrical energy between two circuits through a magnetic field, without any direct electrical connection.
3). What is meant by a transformer’s rating?
A transformer’s rating represents the maximum amount of electrical power it can safely deliver without exceeding the allowable temperature rise in its windings. The temperature limit depends on the insulation class used in the transformer, which defines its thermal tolerance.
4). How and why is a transformer’s rated capacity expressed?
A transformer’s capacity is rated in KVA, not KW. Its rating is often based on the temperature rise.
The losses in the transformer cause the temperature to rise. Copper loss depends on the load current, while iron loss depends on the voltage. Therefore, the total loss in a transformer depends on the volt-ampere (VA) and is not affected by the load power factor.
At any power factor, a given current produces the same I²R loss in a transformer. This loss impacts the machine’s efficiency by generating heat.
The power factor affects the output in kilowatts (kW). When the power factor decreases for a specified kW load, the load current increases proportionally. This higher current leads to increased losses and a rise in the transformer’s operating temperature.
Due to these factors, transformers are rated in kVA instead of kW, as this accounts for the total apparent power (current and voltage) without considering the power factor.
5). What is a transformer power factor?
A transformer’s power factor is very low and lagging under no-load conditions. However, when the transformer is loaded, its power factor closely matches or becomes nearly equal to the power factor of the connected load.
6). In a transformer, what is the normal phase difference between the voltage & the on-load current?
In a transformer, the phase difference between the voltage and the on-load current is about 75°. This occurs because the no-load current consists of two components: the magnetizing current, which lags the applied voltage by 90°, and the core loss current, which is in phase with the applied voltage.
7). What are the main components of a transformer?
The main components of the transformer are as follows.
- Laminated iron core with clamping structures
- Primary winding
- Secondary winding
- Oil-filled insulating tank
- High Tension (H.T) terminals with bushings
- Low Tension (L.T) terminals with bushings
- Conservator tank
- Breather
- Vent pipe
- Winding Temperature Indicator (WTI)
- Oil Temperature Indicator (OTI)
- Radiator
- Neutral CT
- Tap Changer
- Bushings
- Cooling Fans
8). Why is a specific material chosen for transformer cores?
Laminated silicon steel with a silicon content of 4-5% is used due to its high electrical resistance, excellent permeability, non-aging properties, and low iron loss.
9). What is the role of the iron core in a transformer?
The iron core in a transformer provides a low-reluctance path for the magnetic flux for efficient energy transfer between the primary and secondary windings.
10). How is magnetic leakage minimized?
Magnetic leakage in a transformer is minimized by:
- Sectionalizing and Interleaving of windings: The two windings are divided into multiple coils to reduce the flux leakage.
- Using a tightly coupled core design: Ensuring the primary and secondary windings are closely wound around the same core reduces flux leakage.
- Employing laminated cores: Laminations help guide the magnetic flux efficiently within the core.
- Optimizing winding placement: Arranging windings concentrically or in a sandwich configuration ensures better flux linkage.
- Using high-permeability core materials: Materials like silicon steel concentrate the magnetic flux, reducing leakage paths.
- Proper winding insulation: Ensures minimal stray fields by maintaining controlled flux paths.
11). Why should the iron core joints be staggered?
The joints in an iron core should be staggered to reduce magnetic reluctance and minimize flux leakage at the joints. Staggering ensures a smoother magnetic path, improving efficiency and reducing losses caused by discontinuities in the core.
12). Why Does the Transformer Power Factor Remain Low When There is No Load?
The current flowing through a transformer has two components: the magnetizing current (Im), which is 90 degrees out of phase with the applied voltage, and the working current(Iw) in phase with the applied voltage.
Under no-load conditions, the major portion of the excitation current drawn by the transformer is used to magnetize the core. This magnetizing current creates a magnetic field in the transformer circuits (inductive nature).
Therefore, during no-load conditions, the excitation current mainly has a magnetizing current, leading to a low power factor. The transformer power factor under no-load conditions ranges from 0.1 to 0.2.
13). What happens when a DC supply is applied to a transformer?
When a DC supply is applied to the transformer’s primary winding, no back electromotive force (EMF) is induced.
Back EMF is important because it limits the current flow through the transformer, and prevents excessive current from damaging the windings. Without back EMF, the transformer draws a large amount of current, causing the primary windings to overheat and potentially burn out.
Therefore, when a DC supply is applied to a transformer, the primary windings will burn due to excessive current flow.
14). When is the maximum efficiency of a power transformer and a distribution transformer achieved?
When the transformer’s core losses equal the copper losses, the transformer achieves maximum efficiency at a specific load factor
PCopper loss=α2×PCore loss
The optimum efficiency of a transformer is achieved when core loss equals copper loss, as shown in the above equation, for a specific load factor (α). Core losses remain constant regardless of load, while copper losses vary with load. When these two losses are equal, the transformer reaches its maximum efficiency for a given load factor.
The calculation of core losses depends on the transformer’s application. Both core and copper losses are considered equal when optimizing efficiency. Power transformers, which handle bulk power in generating stations and substations, operate close to full load continuously, thus being optimized for maximum efficiency at full load.
On the other hand, the power delivery of distribution transformers varies throughout the day. Consequently, distribution transformers are designed to be most efficient at around 50% of their rated full load.
15). What are the requirements for two transformers to operate in parallel?
To ensure two transformers can operate in parallel, the following requirements must be met:
- Same Voltage Rating: Both transformers must have the same primary and secondary voltage ratings.
- Same Impedance Ratio: The per-unit impedance of two transformers should be the same to maintain a balanced load distribution. Different impedance ratios could lead to unequal loading.
- Phase Compatibility: Both transformers must have the same phase configuration (e.g., both should be Y-Y or Δ-Δ connected).
- Same Frequency: Transformers must operate at the same frequency (e.g., 50 Hz or 60 Hz) to avoid synchronization problems.
- Same Phase Sequence: The phase sequences of the two transformers must be the same.
16). What is the purpose of using silica gel in transformers?
Silica gel is used to absorb moisture from the air that enters the transformer’s conservator.
As the transformer “breathes,” air enters through the breather. This air comes into contact with the heated transformer oil in the conservator and helps convectively dissipate heat. If the incoming air contains moisture, it can degrade the insulating properties of the transformer oil.
To prevent this, silica gel crystals are placed in the breather to absorb moisture from the air. Initially, these silica gel crystals are blue, but as they absorb moisture, they change to pink.
17). What is the function of an isolation transformer?
An isolation transformer provides electrical isolation between the primary and secondary windings and prevents direct electrical connection between the input and output circuits.
18). What is exciting current?
The exciting current refers to the current, or amperes, required for excitation. Most lighting and power transformers have an exciting current ranging from approximately 10% for smaller capacities (around 1 KVA or lower) to about 0.5% to 4% for larger capacities (up to 750 KVA). This exciting current has two components: a real component related to losses(no-load watts), and a reactive component known as kVAR.
19). What are taps & what are their applications?
Some transformers are equipped with taps on the high-voltage winding to adjust for high or low-voltage conditions, ensuring the delivery of full-rated output voltages at the secondary terminals. These taps are labeled as “ANFC” (Above Normal Full Capacity) or “BNFC” (Below Normal Full Capacity) as per ASA and NEMA standards.
In the case of an on-load tap changer, the tap position can be adjusted while the transformer is running. However, for an off-load tap changer, the transformer must be switched off before changing the tap position.
20). In a tap-changing transformer, is the tap linked to the primary or secondary side?
In a tap-changing transformer, the taps are connected to the high-voltage winding because of low current. These taps allow for adjustments in the turns of high-voltage winding, which adjusts the secondary voltage and maintains the desired output. When tapings are connected to the low-voltage side, sparks may occur due to the high current.
21). What is a transformer’s voltage ratio?
.At no load, the voltage ratio is the ratio of the voltage between the line terminals of one winding to the voltage between the line terminals of the other winding.
22). How does a variable frequency transformer work?
A variable frequency transformer (VFT) is used to transfer electricity between two asynchronous alternating current zones. It is a double-fed electric machine, similiar to a vertical shaft hydropower generator, consisting of a three-phase wound rotor connected to an external alternating current power circuit via slip rings. A direct-current torque motor is mounted on the same shaft.
By adjusting the torque applied to the shaft, the direction of the power flow is altered. When no force is applied, the shaft rotates due to a frequency difference between the rotor and stator networks.
As a result, the VFT operates as a continuously variable phase-shifting transformer, enabling the efficient flow of electricity between two interconnected networks.
23). What happens if a DC supply is applied to a transformer’s primary?
Transformers have high inductance and low resistance. However, in a DC supply, there is no inductance, meaning only the resistance affects the electrical circuit.
As a result, a large amount of electrical current flows through the transformer’s primary side. This excess current can cause the coil and insulation to overheat and eventually burn out.
24). Why are Delta-Star Transformers used to power lighting loads?
Lighting loads require a neutral conductor, so the secondary winding must be star-connected. These loads are always uneven across the three phases.
To minimize current imbalance in the primary, a delta winding is used. A delta-star transformer is typically employed for lighting loads.
25). What types of cooling systems are applicable in transformers?
The types of cooling systems applicable in transformers are:
- Oil Natural Air Natural (ONAN)
- Oil Natural Air Forced (ONAF)
- Oil Forced Air Natural (OFAN)
- Oil Forced Air Forced (OFAF)
- Water Cooled (Oil to Water or Direct Water Cooling)
- Air Blast Cooling
- Forced Oil Forced Air (FOFA)