Three-phase unbalance is a common power quality issue in the operation of oil-immersed transformers. It not only reduces transformer efficiency but may also cause local overheating, accelerated insulation aging, and even equipment damage. To address this issue, Xinhong Electrical, drawing on years of experience in transformer manufacturing and operation, proposes the following countermeasures.
Increased transformer losses – Unbalanced currents generate additional losses in the core and windings, raising temperature and shortening transformer life.
Zero‑sequence current – In Yyn0‑connected oil‑immersed transformers, zero‑sequence currents cause local overheating in windings and structural parts.
Unbalanced output voltage – Affects downstream equipment, causing motor vibration, electronic device malfunctions, etc.
The Dyn11 connection is recommended for oil‑immersed transformers. Compared with Yyn0, the Dyn11 connection allows a certain amount of three‑phase unbalanced current to circulate through the delta winding on the low‑voltage side, greatly reducing the influence of zero‑sequence current on the core and tank structure and lowering the risk of local overheating. Xinhong Electrical offers oil‑immersed transformers with Dyn11 and other connection options to suit different operating conditions.
Install SVG (Static Var Generator) or three‑phase load balancing devices on the low‑voltage side. These devices monitor each phase current in real time, dynamically adjust reactive power and active current distribution, and keep the degree of three‑phase unbalance within allowable limits (generally no more than 15%).
Evenly distribute single‑phase loads – In residential or commercial areas, distribute single‑phase loads as evenly as possible among phases A, B, and C, and periodically adjust loads.
Use four‑wire supply – The neutral conductor cross‑section should be at least half that of the phase conductor, and ensure reliable neutral connections to reduce neutral point voltage shift.
Install intelligent monitoring terminals – Monitor three‑phase current, voltage, temperature and other parameters in real time, and set alerts for excessive unbalance.
Perform regular infrared thermography – Check the transformer tank surface, bushings and terminals for local hot spots caused by unbalance.
For substation areas with severe seasonal or cyclic unbalance, consider automatically switched interphase power capacitors or automatic load transfer switches to dynamically adjust load distribution among phases.
According to Chinese national standard GB/T 13462 and electric power industry codes, the permissible three‑phase unbalance for Yyn0‑connected oil‑immersed transformers should not exceed 15%. For Dyn11 connections, the limit may be relaxed but is still recommended to be kept within 20%. In addition, the neutral current should not exceed 25% of the rated phase current.
Xinhong Electrical recommends that when selecting oil‑immersed transformers, users give priority to the Dyn11 connection and equip the system with necessary reactive compensation and load monitoring devices. For existing Yyn0 transformers with serious unbalance problems, consider replacing them with Dyn11 transformers or adding balancing devices on the low‑voltage side. In addition, regular load measurement and three‑phase current analysis are effective ways to prevent transformer failures caused by unbalance.
For further information on solutions for three‑phase unbalance in oil‑immersed transformers, please contact the Xinhong Electrical technical team. We provide full‑process services from transformer selection and monitoring devices to distribution network optimisation.
This article was prepared by the Technical Centre of Xinhong Electrical to help users improve the operational reliability of oil‑immersed transformers.
