Phase Shifting Transformer (Quadrature Booster)

Quiz

Phase-Shifting Transformer is also known as Quadrature Booster or Phase Angle Regulating Transformer or Phase Angle Regular (PAR) or Phase Shifter.

A phase-shifting transformer is a special type of transformer used in power transmission networks for controlling the flow of real-power in the system. It is an important component of a smart grid system, as it provides control over the flow of power in the grid.

Read this chapter to get a good understanding of phase shifting transformer, its structure, working, advantages, and applications.

Theory of Power Flow Control in Transmission Network

In an alternating current transmission network, the power flow in a transmission line is directly proportional to the sine of the difference in phase angle of the voltage between sending end and receiving end.

Therefore, the power flow through the transmission lines can be controlled by manipulating the phase angle. To perform this operation, the phase shifting transformer or quadrature booster is used.

Introduction to Phase Shifting Transformer

A Phase Shifting Transformer (PST) is a specially designed electrical transformer which is employed in three-phase alternating current transmission networks to control the flow of real power through the lines.

This operation of the phase shifting transformer is based on the manipulation of difference of phase angles between sending end and receiving end voltages. The phase shifting transformers are mainly used to distribute total power across the network to prevent overloading of individual lines and to improve the stability and efficiency of the overall transmission network.

In a transmission network, the phase shifting transformer generally performs the following functions −

It allows operators to precisely control the flow of active power through the transmission lines and makes it independent of the power generation.
By rerouting of power through available parallel circuits, it prevents overloading of transmission lines and other equipment.
It also improves the power transmission capacity of the transmission grid.

Structure of Phase Shifting Transformer

A typical phase shifting transformer consists of two separate transformer units namely, a series transformer unit and a shunt transformer unit. These two units are connected together as shown in the following figure.

From this circuit diagram, it can be seen that the primary windings of the shunt transformer are connected across the three-phase transmission lines and the secondary windings are connected to the primary winding of the series transformer. Also, the secondary windings of the series transformer unit are connected in series with the transmission lines.

Working of Phase Shifting Transformer

The main function of a phase shifting transformer is to adjust the phase angle between the input and output voltages in a transmission network. This adjustment helps in controlling the flow of real power in the transmission lines. The detailed working of a phase shifting transformer to perform the control of real power flow is explained below.

The shunt transformer unit has its primary winding connected across the phases of the transmission line, thus it produces an output voltage which is shifted by 90 out of phase (quadrature) with respect to the supply voltage.

The shifted output voltage of the shunt transformer unit is then supplied to the series transformer unit. Because the secondary winding of the series transformer unit is connected in series with the transmission line phases adds the phase shifted component. Therefore, the overall output voltage of the transmission line is the vector sum of the supply voltage and the 90 or quadrature component produced by the shunt transformer.

The magnitude of the quadrature component can be controlled by adjusting the tap connections on the secondary winding of the shunt transformer unit.

Modes of Operation

Depending on the system conditions, the quadrature booster can be operated in the following two modes −

Boost Mode − In this mode of operation, the phase angle between input and output voltages is increased. In this mode, the real power flow towards the load is boosted.
Buck Mode − In this mode, the phase angle is reduced that results in reduced real power flow towards the load.

Functions of Phase Shifting Transformer

Phase shifting transformers are important components in a smart grid transmission system. The main function of a phase shifting transformer is to control and manage the flow of real power in the transmission lines.

Some of the major functions of a phase shifting transformer are explained here −

Power flow control − It is the primary function of a phase shifting transformer. PST controls the flow of real or active power towards the load in a transmission system. This function results in optimization of the transmission line operation and preventing overload conditions.
Load balancing − Phase shifting transformer also helps balancing the load across all the parallel transmission lines to avoid overloading of any individual line in the network.
Adjustment of phase angle difference − Phase shifting transformer adjusts the difference in phase angles between input and output voltages of the transmission line. This provides a control over the flow of active power and enables effective power transmission.
Thermal overload protection − By controlling the power flow and preventing the overloading of power lines, the phase shifting transformer protects the system equipment from overheating and failures.
Effective management of high voltage and high-power systems − Phase shifting transformer can handle large amounts of voltages and powers. Therefore, they are used to manage high voltage power transmission system effectively.

Types of Phase Shifting Transformers

Depending on the construction and operational properties, the phase shifting transformers can be classified into the following main types −

Direct Phase Shifting Transformer (Direct PST)

The following are some important characteristics of the direct PST −

It has a 3-core design.
Its windings are arranged in an appropriate manner to achieve the desired phase shift.
The direct phase shifting transformers has a simple design as compared to others.
It is mainly used in transmission systems that requires a straightforward phase angle adjustment and power flow control without any complexity.

Indirect Phase Shifting Transformer (Indirect PST)

The following points highlight some important characteristics of the indirect phase shifting transformer −

It uses two separate transformer units that are a series transformer unit and a variable tap exciter unit.
The variable tap exciter unit is employed for modifying the magnitude of the quadrature voltage output.
The series transformer unit is employed for adding the quadrature voltage into the supply voltage at an appropriate phase angle.
The indirect phase shifting transformer provides more precise and flexible control of the phase angle, voltage magnitude, and power flow control.
Thus, these transformers are mainly used in complex transmission systems where precise power flow control is critical.

Asymmetrical Phase Shifting Transformer (Asymmetrical PST)

The important characteristics of an asymmetrical phase shifting transformer are listed below −

This phase shifting transformer produces an output voltage which has a different magnitude and phase angle with respect to the supply voltage.
This transformer allows to provide higher adjustment in the characteristics of the output voltage.
The asymmetrical PST is mainly used in industrial and specialized power transmission systems where a specific voltage characteristic is required.

Symmetrical Phase Shifting Transformer (Symmetrical PST)

The following are the important characteristics of the symmetrical phase shifting transformer −

The symmetrical phase shifting transformer produces an output voltage which has an equal magnitude as the supply voltage but has a different phase angle.
As compared to asymmetrical PST, the symmetrical PST has simpler design and high efficiency.
It is primary employed in standard transmission systems that required phase angle adjustment without any change in the line voltage.

Tests of Phase Shifting Transformer

On a phase shifting transformer, we have to perform various tests to determine its health and to ensure its intended functioning. Testing of a phase shifting transformer is an important process to ensure its reliability. In general, some tests are performed in a test laboratory at the manufacturer premises and some other are performed at the installation site.

The following are some common tests that we have to perform on a phase shifting transformer to evaluate its conditions −

Turns Ratio Test − This test is aimed for verification of turns ratio of the transformer as per the design specifications.
Winding Resistance Test − It is performed to determine the resistances of the primary and secondary windings and assess any kind of fault or defect in the windings of the phase shifting transformer.
Phase Shift Test − This test is performed to check whether the phase shifting transformer is producing the desired phase shift or not, between the input and output voltages.
Excitation Current Test − This test is performed on the phase shifting transformer to determine the value of current drawn by the transformer under no-load condition.

Advantages of Phase Shifting Transformer

A phase shifting transformer offers several advantages when used in a power transmission system. Some of the common benefits of using phase shifting transformer in transmission system are listed below −

Phase shifting transformer enables grid operators to manage the loop flows occurred due to interconnected multiple transmission paths. This helps in reducing the inefficiencies and overloading of lines.
Phase shifting transformer can reroute the power flows and prevent heavy loading of certain transmission lines.
Phase shifting transformer also helps in equal distribution of power across all the parallel lines and minimizes the risk of overloading any individual line.
It also improves the overall power flow capacity of the transmission system and reduce the need for installation of new lines.
Phase shifting transformer provides economic ways of improving resource utilization in the transmission systems and reduces the needs for new infrastructure installation and capital required.
Phase shifting transformers improve the reliability of the transmission system through optimized power flow and reduced line overloading.
Phase shifting transformer reduces the risk of thermal overloading of transmission lines and hence improves their lifespan. It also results in minimum requirement of maintenance.

Applications of Phase Shifting Transformer

The following are some common applications of phase shifting transformers in electrical power system −

It is used in transmission system to control the flow of real power in the transmission lines.
It is also employed for phase angle shifting to improve utilization of capacity of the transmission grid.
It is also used to address unpredictable supply issues.
Phase shifting transformers are also used in various advance power system applications like improve the capacity substations, sharing reserve power within substations, improve load management and stability, etc.

Conclusion

A phase shifting transformer or quadrature booster is a specially designed transformer to improve the power flow capacity and stability of the transmission system by adjustment of phase angle between input and output voltage. This transformer is used in smart grids to manage the flow of real-power in transmission networks. It is an important component of a smart grid transmission system because it integrates conventional and renewable energy resources into a single grid.

Lets now move to the next chapter and learn about another important component of the smart grid, i.e., Plug-In Hybrid Electric Vehicle (PHEV).

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