Power transformer engineering reference
Transformer WTI, OTI, Oil-Level Gauge and Cooling-Control Signal Chain
Engineering guide to WTI, OTI, oil-level gauges, local indication, alarm contacts, cooling starts, remote analog signals, SCADA mapping, calibration and FAT.
WTI and OTI have different roles
OTI indicates oil temperature. WTI provides an indication related to winding thermal condition using the design method of the specific instrument or system. The RFQ should define measured quantity, range, accuracy, contact quantity and duty, analog output, installation and remote interface—not merely “temperature indicators included.”
Trace the signal end to end
The chain can include a sensing element, local pointer, converter, terminal blocks, control cabinet, analog input and SCADA point. A disagreement may result from calibration, range, wiring, conversion or software mapping. Changing a display offset before locating the source can hide the actual defect.
Alarms and cooling starts
Temperature instruments commonly provide contacts for alarms and cooling stages. The approved project philosophy should define the use of each contact. A generic assumption that every high-temperature contact must trip the transformer is not appropriate; duties must be reviewed with the owner and protection design.
Oil level and temperature belong together
Conservator oil level changes with oil temperature. Oil-level gauges may also provide high or low contacts. Monitoring should preserve oil temperature, oil level and cooling status together so an isolated alarm is not automatically treated as leakage or overheating.
FAT and commissioning
Inject or simulate the specified input and verify local display, remote analog value, alarm contacts, cooling starts, failure feedback, auxiliary supply and SCADA point name. Record input, indicated value, operating value, reset value and terminal number.
Periodic verification
Temperature instruments should be calibrated under the applicable maintenance program. Infrared thermography can provide external surface evidence but does not replace an internal winding-temperature indication. Load, ambient temperature and cooling state should accompany trend data.
Procurement and FAT hand-in-hand matrix
| Buyer / engineer should define | Supplier should submit | FAT / commissioning should verify |
|---|---|---|
| Instrument data | WTI/OTI type, range, accuracy and installation | Nameplate, range and local indication |
| Contacts | Alarm and cooling-start duties | Operating and reset tests |
| Remote outputs | Signal type, scaling and terminal numbers | Analog value and wiring consistency |
| SCADA mapping | Point names, engineering units and status logic | End-to-end point verification |
Frequently asked questions
Must WTI and OTI show the same value?
No. They represent different thermal quantities, but unexplained disagreement with the approved design still requires investigation.
Can infrared temperature replace WTI?
No. Infrared measurements mainly describe accessible surface conditions and are complementary evidence.
Technical references
- Qin Ganghua, Wang Zhanhong, Wang Xin, et al. 《火力发电企业绝缘技术监督手册》 [Insulation Technical Supervision Manual for Thermal Power Enterprises]. Zhejiang University Press. Used for transformer components, tap changers, oil systems, cooling, grounding and operating inspection.
- Sun Kejun, Jing Chenghao, Wang Zhongjie, et al. 《电工手册》 [Electrical Engineering Handbook]. China Machine Press. Used for oil-immersed construction, pre-energization checks, breathers, cooling equipment and routine inspection.
- Zhou Qiukuan, Liu Mingjun, Liu Yan, et al. 《电力设备带电检测技术及应用》 [Live Detection Technologies and Applications for Power Equipment]. Jinan University Press. Used for DGA, thermography, high-frequency current sensing and live diagnostic correlation.
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Final engineering values, setpoints and operating decisions require written confirmation from the project engineer, owner or utility and manufacturer.