PlantFlow 3D › Guides › Distribution transformer Dyn11 and control transformer explained
Distribution transformer Dyn11 and control transformer explained
What the vector group Dyn11 means, why the LV side has a neutral, what happens when one HV phase is lost, and how a control transformer supplies a safe control voltage.
Reading the vector group Dyn11
The capital D means the high-voltage winding is connected in delta. The small y means the low-voltage winding is in star, and n means its star point is brought out as the neutral. The number 11 means the LV voltages lead the HV voltages by 30° (11 o’clock on a clock face).
Because the LV side is star with a neutral, it supplies 400 V between phases and 230 V from each phase to neutral, which is exactly what buildings and plants need.
Current on both sides
A transformer keeps power roughly the same on both sides, so current scales inversely with voltage: I1 ≈ I2 × V2 / V1. A 100 kVA transformer at 400 V delivers up to about 144 A, while drawing only about 5.2 A at 11 kV. The impedance voltage uk (often 4–6 %) limits the short-circuit current on the LV side.
Losing an HV phase
In a delta–star transformer each LV phase winding sits on the same core as one HV winding connected between two HV phases. If one HV phase is lost, two HV windings lose part of their supply and the LV voltages become unbalanced. Three-phase motors on the LV side then run single-phased and their overload relays must trip them.
The control transformer
A control transformer, for example 400/230 V, takes two phases on its primary H1–H2 and gives an isolated control supply on its secondary. The secondary is protected by its own MCB or fuse and one side is usually earthed. Its VA rating must cover all coils and lamps; contactor coils draw several times more VA when pulling in than when holding, so size it from the manufacturer’s data.
Try it yourself
Open the MCC circuit, choose the multimeter and measure T1.2U–T1.2V (400 V), T1.2U–T1.2N (230 V) and T2.L–T2.N (230 V control).
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.