Y connection is a winding connection of a transformer. In this connection, one end of each winding is connected to a common neutral point and the other end is connected to a power supply or load, thus forming a star-shaped structure.
• A Y-connected transformer usually consists of three windings corresponding to the three phases of the power supply (A, B, C).
• One end of each winding is connected to the neutral point (N) and the other end is connected to the phase line (L1, L2, L3).
• The neutral points of the three windings are connected to form a “Y” shape.
• The phase of the three-phase power supply is distributed at 120 degrees.
1. Neutral grounding:
• The Y-connection provides a clear neutral point that can be easily grounded to improve the safety and stability of the system.
• Neutral grounding can effectively prevent accidents such as leakage and short circuit.
2. Reduce harmonics:
• In the case of three-phase load balancing, Y can reduce the influence of harmonics and improve the power supply quality.
3. Adapt to a variety of loads:
• Y connection is suitable for many types of loads, including linear loads and nonlinear loads, and is highly adaptable.
1. Single-phase fault:
• In a Y connection, if there is a single-phase fault (such as a short circuit), the current will be unbalanced, which may cause overload and burn the device.
2. Ground fault:
• The design of neutral ground may cause an increase in the current of the ground fault, causing damage to the device.
3. Load imbalance:
• If the load is not uniform, it will lead to unstable voltage between phases, which will cause power quality problems.
Phase voltage and line voltage:
Definition
Line voltage/line to line voltage
Line voltage refers to the voltage between phase and phase in a three-phase power system, that is, the voltage between two phase lines (or live lines). In three-phase systems, it is usually labeled 
The type voltage of the transformer is line voltage, such as 2500kVA 11/0.4kV
Phase Voltage
Phase voltage refers to the voltage between the two ends of each phase winding in a three-phase power system, usually expressed by 
• In a Y-connection, the phase voltage (the voltage in each winding) is equal to 
times the line voltage (the voltage between phases).
• Expression formula: 
= line voltage
= phase voltage
Current relationship:
Definition:
Line Current
The current flowing through a phase line (or live line) in a three-phase system, usually represented by 
.
The current in the transformer nameplate is line current
Phase Current
Refers to the current flowing through a single-phase winding
In a Y connection (star connection), refers to the current between the phase line and the neutral point
In a D connection (triangle connection), it refers to the current in a single phase winding. It is usually represented by 
• In a Y-connection, the current flowing through each winding (phase current) is equal to the line current:
• Expression formula: 
= line current
= phase current
D connection (Delta connection, triangle connection, Δ connection)
D connection is a kind of winding connection of transformer. In this connection, the ends of each of the three windings (or three phases) are connected to the starting point of the other winding, thus forming a triangular closed loop, Delta in English or denoted by the symbol ∆
• A D connection transformer is usually composed of three windings corresponding to the three phases of the power supply (A, B, C).
• The ends of the three windings are connected in turn to form a closed triangular structure.
• In the D-connection, the end of the first phase (phase R) winding is connected to the beginning of the second phase (phase Y) winding, the end of the second phase winding is connected to the beginning of the third phase winding, and the end of the third phase winding is connected back to the beginning of the first phase winding, forming a closed loop.
1. Improve short-circuit ability:
• D connection mode can provide a large short circuit capacity, suitable for large load occasions.
2. Neutral ground:
• The D-connected transformer has no neutral point, which relatively reduces the problems caused by poor grounding, and there is no risk of neutral grounding failure.
3. Suitable for high load conditions:
• It can withstand transient high current loads, so it is very effective in the start-up process of large industrial equipment and electric motors.
4. Good power quality:
• D connection can effectively suppress harmonics and perform well in user occasions with high requirements for power quality.
1. Grounding is not allowed:
• Because the D connection has no neutral point, it cannot be grounded effectively, which in some cases can lead to safety hazards.
2. Risk of load imbalance:
• If the load is not balanced, it may cause an imbalance in the three-phase power supply, which in turn causes power quality problems.
3. Adjustment difficulties:
• In terms of load adjustment and voltage regulation, D connection is relatively difficult.
1. Voltage and current relationship:
In a D connection, the line voltage of the three phases is equal to the phase voltage of each winding (that is, the voltage of a specific phase).
Expression formula: 
= line voltage
= phase voltage
• The phase current of each winding is times the line current, that is:
• Expression formula:
= line current
= phase current
2. Phase relationship:
• The difference between phases in the D connection is 120 degrees, the same as in the Y connection, but the relationship between phase current and phase voltage is different from that in the Y connection.
Neutral point
The neutral point of the transformer refers to the common connection point of each phase winding in the Y-connection (star connection) mode. In this connection, one end of the transformer is connected to a common neutral point and the other end is connected to a power supply or load. The neutral point provides an electrical grounding reference.
• Provide grounding reference
The neutral point can be grounded, providing a stable potential reference to ensure the safe operation of the power system.
• Balance voltage
The neutral point helps to balance the voltage in the three-phase system, reduce the influence of asymmetric voltage, and improve the power supply quality of the power system.
• Failure protection
In the event of a single-phase ground fault, the neutral point provides a current loop that helps protect the device to detect and isolate the fault and avoid system damage.
• Load balancing
The neutral point can be connected to the neutral line of the load, helping to achieve load balancing, especially in the case of asymmetric loads.
Connect groups
The Vector Group of a transformer refers to the combination of the connection mode and phase relationship between the transformer windings, and is represented by a combination of letters and numbers, such as Dyn11
• Capital letters
Indicates the primary connection mode, for example, D, Y, and Z.
• Lowercase letters
Indicates the connection mode of the secondary side, for example, d, y, and z.
• Numbers
Represents the phase difference in units of 30 degrees, such as 0, 1, 2… 11.
• For example, Dyn11
D means that the primary side is a Delta connection, y means that the secondary side is a star connection (wye), n means that the secondary side has a neutral point, and 11 means that the phase difference is 330 degrees (30 degrees lag).
• Dyn11
The primary side is D connected, the secondary side is Y connected, and the secondary side voltage phase is 330 degrees ahead of the primary side voltage (that is, 30 degrees behind).
• YNyn0
The primary side is connected to Y, and the secondary side is connected to Y, without phase difference.
• Dyn1
The primary side is D connected, the secondary side is Y connected, and the phase difference between the high pressure side and the low pressure side is 30 degrees.
