A phase-shifting transformer is a type of transformer used in electrical power systems to control the flow of active or real power and balance the loads in the system. It works by delivering a phase-shifted output power with a specific angle from the input power. This means that the phase of the output power can be continuously adjusted while keeping the magnitude (or strength) constant.
One example of a phase-shifting transformer is the Drysdale Phase Shifting Transformer. It works by using a special construction that allows for the continuous adjustment of the phase shift angle. This allows for better control of the power flow in the power system and helps to stabilize the system.
Construction of Phase Shifting Transformer:
The Drysdale phase shift transformer is a special type of transformer that is used to control the power flow in a multi-network power system. It is similar in construction to an induction motor and consists of a stator winding that is kept stationary and a rotor winding that can be moved.
The stator of the phase shift transformer is wound with single or three-phase windings, similar to induction motors. These windings are kept on the stator slots of laminated silicon steel to reduce iron losses.
To produce a rotating magnetic field, the single-phase winding of the phase shift transformer is split into two windings (i.e., as two-phase winding). A phase difference in the magnetic field is produced by using a phase splitting device. This device is a combination of a series-connected variable resistor and capacitor. The resistance and capacitance of the phase splitter are adjusted until the winding in which it is connected produces a current that is displaced at 90° from the other winding.
The two stator windings are arranged perpendicular to each other (90° apart) and produce a uniform rotating magnetic field. The rotor winding is wound on laminated structured slots, and to reduce leakage reactance, it is kept closer to the stator winding. The rotor rotation causes a phase displacement in the induced emf, which is proportional to the desired angle of phase shift.
A pointer is attached to the top of the rotor and rotates over a scale that indicates the angle of phase displacement. The scale contains different angles of phase displacement so that the required phase shift from the output can be obtained.
Working of Phase Shifting Transformer:
A phase-shifting transformer is a type of transformer that is used to change the phase of an electrical current. It works by using a two-phase stator winding that is displaced at 90 degrees. When a single-phase supply is given to this winding, it produces a uniform rotating magnetic field. This field then links with the rotor conductors, which are arranged in a specific way to allow for a variable phase shift.
Phaser diagram and equivalent circuit of phase shifting transformer are given below:
From fig (2) and phaser diagram following equation obtained
We get the following equation by equating real and imaginary parts.
The induced emf in each of the two stator windings of a phase shifting transformer is proportional to the cosine of the angular displacement from the position of maximum emf.
The emf induced in the rotor winding due to the first stator winding is given by
e1 = KI sinωt cosθ.
This equation represents the voltage induced in the rotor winding due to the first stator winding,
where KI is a constant,
ω is the angular frequency,
t is time and
θ is the angular displacement of the rotor from the position of maximum emf.
Similarly, the emf induced in the rotor winding due to the second stator winding is given by
e2 = KI sin (ωt + 90) cos(θ + 90)
e2 = -KI cos ωt sinθ.
This equation represents the voltage induced in the rotor winding due to the second stator winding.
The resultant emf in the rotor is given by
e = e1 + e2
= KI sinωt cosθ - KI cosωt sinθ.
= KI(sinωt cosθ - cosωt sinθ)
This equation represents the total voltage induced in the rotor winding, which is the sum of the voltages induced by the first and second stator windings.
We can simplify the equation for the resultant emf by using the trigonometric identity
sin (A-B) = sin A cos B - cos A sin B.
This identity is used to transform the expression
e = KI(sinωt cosθ - cosωt sinθ)
= KI sin (ωt - θ).
From the above relation, it can be seen that the emf induced in the rotor has a constant amplitude but its phase depends on the position of rotor i.e. an angle θ.
The circuit of a two-phase stator winding with variable resistance and capacitance is shown in the diagram, along with the corresponding phasor diagram. This type of connection is called a quadrature winding arrangement. The rotor winding is arranged in such a way that the change in phase shift does not affect the magnitude of the induced emf. The phase shift depends on the rotor position (θ).
The rotor position can be adjusted to achieve a required variable phase shift from the rotor output quantities. This means that the transformer can be adjusted to change the phase of the current without affecting the overall strength of the current. This is important in many applications, such as power transmission and distribution, where the phase of the current must be adjusted to match the phase of other electrical components in the system.
Three Phase Shifting Transformer:
A three-phase shifting transformer is a type of transformer that is used to change the phase of a three-phase electrical current. These transformers are similar to single-phase shifting transformers, but are less sensitive to variations in frequency. This means that they can maintain a consistent phase shift even if the frequency of the current changes.
When a three-phase power supply is given to the phase shift transformer, the rotor is rotated to a desired angle to obtain a phase-shifted output without changing the magnitude of the voltage. This is shown in the phasor diagram of the phase-shifted output from the three-phase supply input.
In order to obtain precise phase shifting, the rotor displacement is varied in discrete steps by an external motor. This means that the rotor is not continuously rotated, but rather is moved in small, precise increments to achieve the desired phase shift. This allows for greater control and accuracy in the phase shifting process.
The advantages of phase shifting transformer are:
- They improve the overall reliability and efficiency of electrical power grids.
- They are considered to be one of the most reliable and cost-effective solutions for controlling the flow of power in electrical grids.
- They can be used to adjust the real power flowing through the grid and balance the load on the grid to prevent power outages.
- They can be used to add a circulating current into the network to control load current and prevent power outages.
- They can be used to balance loads and improve the overall performance of the grid.
- They can be used to control the demand for power in an electrical grid and prevent power outages.