Two-Winding and Three-Winding Transformers
A two-winding transformer consists of two windings, meaning it has two voltage levels, such as 110/10kV. The power distribution between the two windings is typically 100%/100%. A three-winding transformer has three windings, meaning it has three voltage levels, such as 110/35/10kV. In this case, the power distribution of the secondary windings does not necessarily need to be 100% for each. For example, it could be 100%/100%/50% (with the 10kV winding having a maximum capacity of 50% of the transformer's total capacity), or 100%/100%/100%. Other than this difference, the two types of transformers are generally the same.
1. Structure and Applications
A three-winding transformer has three windings per phase. When one winding is connected to an AC power supply, the other two windings induce different voltages. This type of transformer is used when a load requires two different voltage levels.
Power plants and substations often need to work with three different voltage levels, so three-winding transformers are widely used in power systems. Each phase's high, medium, and low voltage windings are mounted on the same core pillar. To ensure proper insulation, the high voltage winding is typically placed in the outermost layer, while the medium and low voltage windings are placed in the inner layers.
The rated capacity refers to the capacity of the largest winding. The capacity percentages are commonly in the following forms: 100/100/50, 100/50/100, or 100/100/100.
2. Characteristics
The three ratios of a three-winding transformer are as follows:
k12 = N1/N2 ≈ U1/U2
k13 = N1/N3 ≈ U1/U3
k23 = N2/N3 ≈ U2/U3
When the transformer is under load, ignoring no-load current (I0), the magnetic balance equation is:
I1N1 + I2N2 + I3N3 = 0
I1 + I2/k12 + I3/k13 = 0
I1 + I2' + I3' = 0
In the simplified equivalent circuit:
Z1 = R1 + jX1 is the primary side impedance.
Z2' = R2' + jX2' is the secondary side impedance, referred to the primary side.
Z3' = R3' + jX3' is the third side impedance, referred to the primary side.
These six parameters can be determined through short-circuit tests:
Zk12 = Rk12 + jXk12 = (R1 + R2') + j(X1 + X2')
Zk13 = Rk13 + jXk13 = (R1 + R3') + j(X1 + X3')
Zk23' = Rk23' + jXk23' = (R2' + R3') + j(X2' + X3')
The values for R1, X1, R2', X2', R3', X3' can be calculated as:
R1 = 1/2 (Rk12 + Rk13 - Rk23')
X1 = 1/2 (Xk12 + Xk13 - Xk23')
R2' = 1/2 (Rk12 + Rk23' - Rk13)
X2' = 1/2 (Xk12 + Xk23' - Xk13)
R3' = 1/2 (Rk13 + Rk23' - Rk12)
X3' = 1/2 (Xk13 + Xk23' - Xk12)
Once these parameters are known, the transformer's characteristics can be calculated based on the equivalent circuit.
Both single-phase and three-phase systems can be made into two-winding or three-winding transformers. However, three-phase transformers are more commonly used in power systems. Both types of transformers have broad applications.
3. Autotransformer
What you mentioned as a "single-winding smart transformer" likely refers to an autotransformer. In an autotransformer, the primary and secondary windings are not only magnetically coupled but also electrically connected. This means the low-voltage winding is actually a part of the high-voltage winding, making it seem as though the autotransformer has a single unified winding. The term "autotransformer" implies that the common portion and the series portion of this single winding are magnetically coupled to transmit electrical energy.
In actual design and manufacturing, small-capacity voltage regulators are typically made with a single coil. For larger capacities, even in autotransformers, the windings are not made as a single winding. Although, in theory, it's one winding, in practice, they are still made into two separate windings. This ensures better balance of turns and reduces the risk of damage if the transformer experiences a short circuit during operation.