Electrical Substation Equipment: Power Transformer and Its Functions

A power transformer is a crucial component in electrical substations that plays a fundamental role in the generation, transmission, and distribution of electrical energy. Its primary function is to transform voltage levels, typically stepping them up for long-distance transmission or stepping them down for distribution to end users. Here are the key functions and roles of power transformers in electrical substations:

Power Transformer and Its Functions in Electrical Substations

A power transformer is a static electrical device used in electrical power systems to transfer electrical energy between two or more circuits through electromagnetic induction. Its primary purpose is to change the voltage level of alternating current (AC) electrical energy while keeping the frequency unchanged. This voltage transformation is crucial for the efficient transmission and distribution of electrical power over long distances and for the safe utilization of electricity in various applications.

Power Transformers work as Voltage and Current Transformation Devices in Electrical Substation 

A power transformer works by using electromagnetic induction to change the voltage level of AC electricity. The turn ratio between the primary and secondary coils determines whether the transformer steps up or steps down the voltage. This voltage transformation is crucial for transmitting electricity over long distances, matching the voltage requirements of different electrical devices, and ensuring efficient and safe power distribution.

Voltage Transformation:

A power transformer is an electrical device used to transfer electrical energy between two or more circuits through electromagnetic induction. One of its primary functions is to change the voltage level of alternating current (AC) electricity. It does this by employing the principles of electromagnetic induction, which were first discovered by Michael Faraday in the 19th century.

Here's how a power transformer works as a voltage transformer:

• Basic Components: A typical power transformer consists of two coils of wire, known as the primary coil and the secondary coil, wound around a common iron core. The primary coil is connected to the source of electrical power, while the secondary coil is connected to the load or the electrical device that needs a different voltage level.
• AC Input: When alternating current (AC) flows through the primary coil, it generates a magnetic field around the iron core. The AC voltage is constantly changing direction, which leads to the expansion and contraction of this magnetic field.
• Electromagnetic Induction: According to Faraday's law of electromagnetic induction, a changing magnetic field induces a voltage in a nearby coil. In this case, the changing magnetic field generated by the primary coil induces a voltage in the secondary coil.
• Turns Ratio: The key to voltage transformation in a power transformer is the turn ratio between the primary and secondary coils. The turns ratio is the ratio of the number of turns of wire in the primary coil to the number of turns in the secondary coil. It determines how much the voltage is transformed.

👉If the secondary coil has fewer turns than the primary coil, the output voltage will be lower than the input voltage (step-down transformer).

👉If the secondary coil has more turns than the primary coil, the output voltage will be higher than the input voltage (step-up transformer).

👉If the turns are equal, the transformer will provide a 1:1 voltage ratio, and there will be no voltage transformation.

Current Transformation: 

While primarily used for voltage transformation, transformers can also transform current levels. This feature is essential in various applications, such as impedance matching and connecting systems with different current ratings.

Current transformation in a power transformer refers to the process of changing the magnitude of an electrical current from one level to another. Power transformers are essential components in electrical systems, and one of their primary functions is to transfer electrical energy from one circuit to another while maintaining the same frequency but often at different voltage and current levels.

Here's how current transformation works in a power transformer:

• Primary and Secondary Windings: A power transformer consists of two sets of windings: the primary winding and the secondary winding. The primary winding is connected to the source of electrical power, while the secondary winding is connected to the load or the circuit that requires the transformed electrical energy.
• Turns Ratio: The current transformation ratio is determined by the turn ratio of the windings. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. It is usually expressed as Np/Ns, where Np is the number of turns in the primary winding, and Ns is the number of turns in the secondary winding.
• Current Transformation: According to the turns ratio, the transformer alters the current magnitude between the primary and secondary windings. The relationship is inversely proportional to the turn ratio. Specifically, if the turns ratio is such that Np is greater than Ns (more turns in the primary), the secondary current will be lower than the primary current. Conversely, if Ns is greater than Np, the secondary current will be higher than the primary current.
• Conservation of Power: It's important to note that power is conserved in a transformer. This means that while the current may change, the product of voltage and current (power) remains the same (neglecting losses due to resistance and other factors). This is expressed by the equation:

Vp * Ip = Vs * Is

Where:

Vp is the voltage on the primary side.

Ip is the current on the primary side.

Vs is the voltage on the secondary side.

Is is the current on the secondary side.

Power Transformer Works as Isolation Device in Electrical Substation

Transformers provide electrical isolation between the primary and secondary windings. This isolation helps protect the electrical system by preventing faults or disturbances on one side from affecting the other side.

A power transformer can work as an isolation transformer by providing electrical isolation between its input (primary winding) and output (secondary winding) circuits. Electrical isolation means that there is no direct electrical connection between the primary and secondary windings of the transformer. This isolation serves several important purposes:

Safety: Isolation transformers are commonly used in situations where there is a need to protect people and equipment from electrical shock. By isolating the input and output circuits, any faults or electrical disturbances on one side of the transformer do not directly affect the other side, reducing the risk of electric shock and fire hazards.

Here's how a power transformer achieves electrical isolation:

• Physical Separation: The primary and secondary windings of the transformer are physically separated by an insulating material, typically paper, oil, or other dielectric materials. This physical separation ensures that there is no direct metallic connection between the windings.
• Magnetic Coupling: The primary and secondary windings are wound around a common magnetic core. When an alternating current flows through the primary winding, it creates a magnetic field in the core. This magnetic field induces a voltage in the secondary winding, which is then transferred to the secondary circuit. Importantly, there is no direct electrical connection between the windings; the transfer of energy occurs through magnetic coupling.
• In summary, a power transformer works as an isolation transformer by providing electrical separation between its primary and secondary windings, ensuring safety, voltage transformation, and isolation of electrical disturbances. This isolation is achieved through physical separation and magnetic coupling, allowing energy transfer without a direct electrical connection.

Power Transformer Work as Impedance Matching in Electrical Substation

Transformers are used to match the impedance (resistance to the flow of electric current) between the source and load. This ensures maximum power transfer efficiency and minimizes losses.

Power transformers can be used as impedance-matching devices in electrical circuits. To understand how they work for impedance matching, it's important to know the basic principles of transformers and impedance.

Transformer Basics:

• A power transformer consists of two coils of wire, called the primary and secondary windings, wrapped around a common core.
• When an alternating current (AC) voltage is applied to the primary winding, it creates a magnetic field in the core.
• This magnetic field induces a voltage in the secondary winding, based on the turns ratio between the primary and secondary windings.

Impedance:

• Impedance in an electrical circuit is a measure of opposition to the flow of alternating current. It includes both resistance (R) and reactance (X).
• Resistance (R) represents the real part of the impedance and is due to the resistance of the conductor.
• Reactance (X) represents the imaginary part of the impedance and is due to the inductance (L) and capacitance (C) of the circuit.

Impedance Matching:

• Impedance matching is a technique used to ensure maximum power transfer between two electrical components or circuits.
• When the source impedance matches the load impedance, the maximum power is transferred from the source to the load, minimizing reflections and losses.

Now, let's see how power transformers achieve impedance matching:

Step 1: Determine Source and Load Impedances

Before using a power transformer for impedance matching, you need to know the source impedance (Z_source) and the load impedance (Z_load). These impedances are typically given as a combination of resistance (R) and reactance (X).

Step 2: Calculate the Turns Ratio

The turns ratio (N) of a transformer is the ratio of the number of turns in the primary winding (N_primary) to the number of turns in the secondary winding (N_secondary). It's calculated as:

N=Nsecondary​Nprimary​​

Step 3: Determine Desired Impedance Transformation

To achieve impedance matching, you want to transform the load impedance (Z_load) to match the source impedance (Z_source). The turns ratio N is chosen such that:

N=Zsource​Zload​​​

Step 4: Use the Transformer

Once you've determined the turn ratio N, you can use a transformer with the appropriate winding configuration to achieve the desired impedance transformation. When the transformer is properly matched, the load impedance seen by the source will appear as the source impedance, maximizing power transfer.

In summary, power transformers work for impedance matching by changing the voltage and current levels between the source and load while ensuring that the load impedance is transformed to match the source impedance. This ensures efficient power transfer and minimizes reflections and losses in the electrical circuit.

Power Transformer Work for Phase Shifting 

A power transformer can work as a phase-shifting transformer by altering the relative phase angle between its primary and secondary voltages. Phase-shifting transformers are commonly used in power systems to control power flow, regulate voltage levels, and maintain system stability.

Here's how a power transformer accomplishes phase shifting:

Transformer Construction: A phase-shifting transformer is typically a special type of two-winding transformer. It has two separate windings, the primary and secondary, just like a standard power transformer.

Connection Configuration: The key to achieving phase shifting lies in the connection of the windings. There are two primary methods for achieving phase shift:

• Delta-Wye (Δ-Y) Configuration: In this configuration, the primary winding is connected in the delta (Δ) configuration, and the secondary winding is connected in the wye (Y) configuration. This setup inherently introduces a phase shift between the primary and secondary voltages. The phase shift is usually around 30 degrees or 30°.
• Wye-Delta (Y-Δ) Configuration: In this configuration, the primary winding is connected in the wye (Y) configuration, and the secondary winding is connected in the delta (Δ) configuration. This arrangement also introduces a phase shift between the primary and secondary voltages, typically around -30 degrees or -30°, which is the opposite direction of the Δ-Y configuration.

Phase Angle Control: The amount of phase shift can be adjusted by modifying the transformer's connection. By changing taps on the transformer's windings or using additional equipment like phase-shifting switches or tap changers, you can control the magnitude and direction of the phase shift.

Applications of Phase Shifting Power Transformer:

Voltage Regulation: Phase-shifting transformers are often used to control the voltage levels at different points in the power grid. By changing the phase angle, you can adjust the voltage level at the secondary side relative to the primary side.

• Power Flow Control: They can be employed to control the direction and magnitude of power flow in transmission lines. By changing the phase angle, you can divert power to different paths in the grid as needed.
• Transformer Ratings: Phase-shifting transformers are designed with specific ratings to handle the desired phase-shifting capabilities. The voltage and current ratings must be chosen carefully to ensure that the transformer can handle the power flow and voltage regulation requirements.

In summary, a power transformer can function as a phase-shifting transformer by configuring its windings in specific ways (Δ-Y or Y-Δ) and adjusting the phase shift as needed through tap changers or other control mechanisms. This allows for control of voltage levels and power flow in the electrical grid, contributing to efficient and stable power transmission and distribution

Load Balancing: In distribution substations, transformers help balance the load by distributing electrical power to various feeders or circuits based on demand. This prevents overloading and ensures a more reliable power supply.

Voltage Regulation: Transformers equipped with tap changers allow for voltage regulation. By adjusting the turn ratio, transformers can compensate for voltage fluctuations and maintain stable voltage levels within predefined limits.

Fault Current Limiting: Transformers can limit the fault current during short circuits or faults in the electrical system. This helps protect the system's components and enhances overall safety.

Isolation from Harmonics: Transformers can help mitigate the effects of harmonics in the electrical system, which can be caused by nonlinear loads like variable frequency drives or electronic equipment.

Noise Reduction: Transformers can reduce the noise generated by electrical equipment, which is especially important in urban areas where noise pollution is a concern.

Grounding: Transformers can provide a grounding point for the electrical system, ensuring safety by redirecting fault currents to the ground.

Backup Power: In some applications, transformers are used to connect backup power sources, such as generators or energy storage systems, to the grid or distribution system.

Overall, power transformers are indispensable components in electrical substations, enabling efficient and reliable electrical energy transmission and distribution while ensuring safety and system stability. They are designed with various specifications and features to meet the specific requirements of different substations and electrical systems.